basic.lean

/-
Copyright (c) 2014 Jeremy Avigad. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Jeremy Avigad, Leonardo de Moura
-/
import order.symm_diff
import logic.function.iterate

/-!
# Basic properties of sets

> THIS FILE IS SYNCHRONIZED WITH MATHLIB4.
> Any changes to this file require a corresponding PR to mathlib4.

Sets in Lean are homogeneous; all their elements have the same type. Sets whose elements
have type `X` are thus defined as `set X := X → Prop`. Note that this function need not
be decidable. The definition is in the core library.

This file provides some basic definitions related to sets and functions not present in the core
library, as well as extra lemmas for functions in the core library (empty set, univ, union,
intersection, insert, singleton, set-theoretic difference, complement, and powerset).

Note that a set is a term, not a type. There is a coercion from `set α` to `Type*` sending
`s` to the corresponding subtype `↥s`.

See also the file `set_theory/zfc.lean`, which contains an encoding of ZFC set theory in Lean.

## Main definitions

Notation used here:

-  `f : α → β` is a function,

-  `s : set α` and `s₁ s₂ : set α` are subsets of `α`

-  `t : set β` is a subset of `β`.

Definitions in the file:

* `nonempty s : Prop` : the predicate `s ≠ ∅`. Note that this is the preferred way to express the
  fact that `s` has an element (see the Implementation Notes).

* `subsingleton s : Prop` : the predicate saying that `s` has at most one element.

* `nontrivial s : Prop` : the predicate saying that `s` has at least two distinct elements.

* `inclusion s₁ s₂ : ↥s₁ → ↥s₂` : the map `↥s₁ → ↥s₂` induced by an inclusion `s₁ ⊆ s₂`.

## Notation

* `sᶜ` for the complement of `s`

## Implementation notes

* `s.nonempty` is to be preferred to `s ≠ ∅` or `∃ x, x ∈ s`. It has the advantage that
the `s.nonempty` dot notation can be used.

* For `s : set α`, do not use `subtype s`. Instead use `↥s` or `(s : Type*)` or `s`.

## Tags

set, sets, subset, subsets, union, intersection, insert, singleton, complement, powerset

-/

/-! ### Set coercion to a type -/

open function

universes u v w x

namespace set

variables {α : Type*} {s t : set α}

instance : has_le (set α) := ⟨λ s t, ∀ ⦃x⦄, x ∈ s → x ∈ t⟩
instance : has_subset (set α) := ⟨(≤)⟩

instance {α : Type*} : boolean_algebra (set α) :=
{ sup := λ s t, {x | x ∈ s ∨ x ∈ t},
  le  := (≤),
  lt  := λ s t, s ⊆ t ∧ ¬t ⊆ s,
  inf := λ s t, {x | x ∈ s ∧ x ∈ t},
  bot := ∅,
  compl := λ s, {x | x ∉ s},
  top := univ,
  sdiff := λ s t, {x | x ∈ s ∧ x ∉ t},
  .. (infer_instance : boolean_algebra (α → Prop)) }

instance : has_ssubset (set α) := ⟨(<)⟩
instance : has_union (set α) := ⟨(⊔)⟩
instance : has_inter (set α) := ⟨(⊓)⟩

@[simp] lemma top_eq_univ : (⊤ : set α) = univ := rfl
@[simp] lemma bot_eq_empty : (⊥ : set α) = ∅ := rfl
@[simp] lemma sup_eq_union : ((⊔) : set α → set α → set α) = (∪) := rfl
@[simp] lemma inf_eq_inter : ((⊓) : set α → set α → set α) = (∩) := rfl
@[simp] lemma le_eq_subset : ((≤) : set α → set α → Prop) = (⊆) := rfl
@[simp] lemma lt_eq_ssubset : ((<) : set α → set α → Prop) = (⊂) := rfl

lemma le_iff_subset : s ≤ t ↔ s ⊆ t := iff.rfl
lemma lt_iff_ssubset : s < t ↔ s ⊂ t := iff.rfl

alias le_iff_subset ↔ _root_.has_le.le.subset _root_.has_subset.subset.le
alias lt_iff_ssubset ↔ _root_.has_lt.lt.ssubset _root_.has_ssubset.ssubset.lt

/-- Coercion from a set to the corresponding subtype. -/
instance {α : Type u} : has_coe_to_sort (set α) (Type u) := ⟨λ s, {x // x ∈ s}⟩

instance pi_set_coe.can_lift (ι : Type u) (α : Π i : ι, Type v) [ne : Π i, nonempty (α i)]
  (s : set ι) :
  can_lift (Π i : s, α i) (Π i, α i) (λ f i, f i) (λ _, true) :=
pi_subtype.can_lift ι α s

instance pi_set_coe.can_lift' (ι : Type u) (α : Type v) [ne : nonempty α] (s : set ι) :
  can_lift (s → α) (ι → α) (λ f i, f i) (λ _, true) :=
pi_set_coe.can_lift ι (λ _, α) s

end set

section set_coe

variables {α : Type u}

theorem set.coe_eq_subtype (s : set α) : ↥s = {x // x ∈ s} := rfl

@[simp] theorem set.coe_set_of (p : α → Prop) : ↥{x | p x} = {x // p x} := rfl

@[simp] theorem set_coe.forall {s : set α} {p : s → Prop} :
  (∀ x : s, p x) ↔ (∀ x (h : x ∈ s), p ⟨x, h⟩) :=
subtype.forall

@[simp] theorem set_coe.exists {s : set α} {p : s → Prop} :
  (∃ x : s, p x) ↔ (∃ x (h : x ∈ s), p ⟨x, h⟩) :=
subtype.exists

theorem set_coe.exists' {s : set α} {p : Π x, x ∈ s → Prop} :
  (∃ x (h : x ∈ s), p x h) ↔ (∃ x : s, p x x.2) :=
(@set_coe.exists _ _ $ λ x, p x.1 x.2).symm

theorem set_coe.forall' {s : set α} {p : Π x, x ∈ s → Prop} :
  (∀ x (h : x ∈ s), p x h) ↔ (∀ x : s, p x x.2) :=
(@set_coe.forall _ _ $ λ x, p x.1 x.2).symm

@[simp] theorem set_coe_cast : ∀ {s t : set α} (H' : s = t) (H : ↥s = ↥t) (x : s),
  cast H x = ⟨x.1, H' ▸ x.2⟩
| s _ rfl _ ⟨x, h⟩ := rfl

theorem set_coe.ext {s : set α} {a b : s} : (↑a : α) = ↑b → a = b :=
subtype.eq

theorem set_coe.ext_iff {s : set α} {a b : s} : (↑a : α) = ↑b ↔ a = b :=
iff.intro set_coe.ext (assume h, h ▸ rfl)

end set_coe

/-- See also `subtype.prop` -/
lemma subtype.mem {α : Type*} {s : set α} (p : s) : (p : α) ∈ s := p.prop

/-- Duplicate of `eq.subset'`, which currently has elaboration problems. -/
lemma eq.subset {α} {s t : set α} : s = t → s ⊆ t := eq.subset'

namespace set

variables {α : Type u} {β : Type v} {γ : Type w} {ι : Sort x} {a b : α} {s s₁ s₂ t t₁ t₂ u : set α}

instance : inhabited (set α) := ⟨∅⟩

@[ext]
theorem ext {a b : set α} (h : ∀ x, x ∈ a ↔ x ∈ b) : a = b :=
funext (assume x, propext (h x))

theorem ext_iff {s t : set α} : s = t ↔ ∀ x, x ∈ s ↔ x ∈ t :=
⟨λ h x, by rw h, ext⟩

@[trans] theorem mem_of_mem_of_subset {x : α} {s t : set α} (hx : x ∈ s) (h : s ⊆ t) : x ∈ t := h hx

lemma forall_in_swap {p : α → β → Prop} :
  (∀ (a ∈ s) b, p a b) ↔ ∀ b (a ∈ s), p a b :=
by tauto

/-! ### Lemmas about `mem` and `set_of` -/

lemma mem_set_of {a : α} {p : α → Prop} : a ∈ {x | p x} ↔ p a := iff.rfl

/-- If `h : a ∈ {x | p x}` then `h.out : p x`. These are definitionally equal, but this can
nevertheless be useful for various reasons, e.g. to apply further projection notation or in an
argument to `simp`. -/
lemma _root_.has_mem.mem.out {p : α → Prop} {a : α} (h : a ∈ {x | p x}) : p a := h

theorem nmem_set_of_iff {a : α} {p : α → Prop} : a ∉ {x | p x} ↔ ¬ p a := iff.rfl

@[simp] theorem set_of_mem_eq {s : set α} : {x | x ∈ s} = s := rfl

theorem set_of_set {s : set α} : set_of s = s := rfl

lemma set_of_app_iff {p : α → Prop} {x : α} : {x | p x} x ↔ p x := iff.rfl

theorem mem_def {a : α} {s : set α} : a ∈ s ↔ s a := iff.rfl

lemma set_of_bijective : bijective (set_of : (α → Prop) → set α) := bijective_id

@[simp] theorem set_of_subset_set_of {p q : α → Prop} :
  {a | p a} ⊆ {a | q a} ↔ (∀a, p a → q a) := iff.rfl

lemma set_of_and {p q : α → Prop} : {a | p a ∧ q a} = {a | p a} ∩ {a | q a} := rfl

lemma set_of_or {p q : α → Prop} : {a | p a ∨ q a} = {a | p a} ∪ {a | q a} := rfl

/-! ### Subset and strict subset relations -/

instance : is_refl (set α) (⊆) := has_le.le.is_refl
instance : is_trans (set α) (⊆) := has_le.le.is_trans
instance : is_antisymm (set α) (⊆) := has_le.le.is_antisymm
instance : is_irrefl (set α) (⊂) := has_lt.lt.is_irrefl
instance : is_trans (set α) (⊂) := has_lt.lt.is_trans
instance : is_asymm (set α) (⊂) := has_lt.lt.is_asymm
instance : is_nonstrict_strict_order (set α) (⊆) (⊂) := ⟨λ _ _, iff.rfl⟩

-- TODO(Jeremy): write a tactic to unfold specific instances of generic notation?
lemma subset_def : (s ⊆ t) = ∀ x, x ∈ s → x ∈ t := rfl
lemma ssubset_def : s ⊂ t = (s ⊆ t ∧ ¬ t ⊆ s) := rfl

@[refl] theorem subset.refl (a : set α) : a ⊆ a := assume x, id
theorem subset.rfl {s : set α} : s ⊆ s := subset.refl s

@[trans] theorem subset.trans {a b c : set α} (ab : a ⊆ b) (bc : b ⊆ c) : a ⊆ c := λ x h, bc $ ab h

@[trans] theorem mem_of_eq_of_mem {x y : α} {s : set α} (hx : x = y) (h : y ∈ s) : x ∈ s :=
hx.symm ▸ h

theorem subset.antisymm {a b : set α} (h₁ : a ⊆ b) (h₂ : b ⊆ a) : a = b :=
set.ext $ λ x, ⟨@h₁ _, @h₂ _⟩

theorem subset.antisymm_iff {a b : set α} : a = b ↔ a ⊆ b ∧ b ⊆ a :=
⟨λ e, ⟨e.subset, e.symm.subset⟩, λ ⟨h₁, h₂⟩, subset.antisymm h₁ h₂⟩

-- an alternative name
theorem eq_of_subset_of_subset {a b : set α} : a ⊆ b → b ⊆ a → a = b := subset.antisymm

theorem mem_of_subset_of_mem {s₁ s₂ : set α} {a : α} (h : s₁ ⊆ s₂) : a ∈ s₁ → a ∈ s₂ := @h _

theorem not_mem_subset (h : s ⊆ t) : a ∉ t → a ∉ s :=
mt $ mem_of_subset_of_mem h

theorem not_subset : (¬ s ⊆ t) ↔ ∃a ∈ s, a ∉ t := by simp only [subset_def, not_forall]

/-! ### Definition of strict subsets `s ⊂ t` and basic properties. -/

protected theorem eq_or_ssubset_of_subset (h : s ⊆ t) : s = t ∨ s ⊂ t :=
eq_or_lt_of_le h

lemma exists_of_ssubset {s t : set α} (h : s ⊂ t) : (∃x∈t, x ∉ s) :=
not_subset.1 h.2

protected lemma ssubset_iff_subset_ne {s t : set α} : s ⊂ t ↔ s ⊆ t ∧ s ≠ t :=
@lt_iff_le_and_ne (set α) _ s t

lemma ssubset_iff_of_subset {s t : set α} (h : s ⊆ t) : s ⊂ t ↔ ∃ x ∈ t, x ∉ s :=
⟨exists_of_ssubset, λ ⟨x, hxt, hxs⟩, ⟨h, λ h, hxs $ h hxt⟩⟩

protected lemma ssubset_of_ssubset_of_subset {s₁ s₂ s₃ : set α} (hs₁s₂ : s₁ ⊂ s₂)
  (hs₂s₃ : s₂ ⊆ s₃) :
  s₁ ⊂ s₃ :=
⟨subset.trans hs₁s₂.1 hs₂s₃, λ hs₃s₁, hs₁s₂.2 (subset.trans hs₂s₃ hs₃s₁)⟩

protected lemma ssubset_of_subset_of_ssubset {s₁ s₂ s₃ : set α} (hs₁s₂ : s₁ ⊆ s₂)
  (hs₂s₃ : s₂ ⊂ s₃) :
  s₁ ⊂ s₃ :=
⟨subset.trans hs₁s₂ hs₂s₃.1, λ hs₃s₁, hs₂s₃.2 (subset.trans hs₃s₁ hs₁s₂)⟩

theorem not_mem_empty (x : α) : ¬ (x ∈ (∅ : set α)) := id

@[simp] theorem not_not_mem : ¬ (a ∉ s) ↔ a ∈ s := not_not

/-! ### Non-empty sets -/

/-- The property `s.nonempty` expresses the fact that the set `s` is not empty. It should be used
in theorem assumptions instead of `∃ x, x ∈ s` or `s ≠ ∅` as it gives access to a nice API thanks
to the dot notation. -/
protected def nonempty (s : set α) : Prop := ∃ x, x ∈ s

@[simp] lemma nonempty_coe_sort {s : set α} : nonempty ↥s ↔ s.nonempty := nonempty_subtype

alias nonempty_coe_sort ↔ _ nonempty.coe_sort

lemma nonempty_def : s.nonempty ↔ ∃ x, x ∈ s := iff.rfl

lemma nonempty_of_mem {x} (h : x ∈ s) : s.nonempty := ⟨x, h⟩

theorem nonempty.not_subset_empty : s.nonempty → ¬(s ⊆ ∅)
| ⟨x, hx⟩ hs := hs hx

/-- Extract a witness from `s.nonempty`. This function might be used instead of case analysis
on the argument. Note that it makes a proof depend on the `classical.choice` axiom. -/
protected noncomputable def nonempty.some (h : s.nonempty) : α := classical.some h

protected lemma nonempty.some_mem (h : s.nonempty) : h.some ∈ s := classical.some_spec h

lemma nonempty.mono (ht : s ⊆ t) (hs : s.nonempty) : t.nonempty := hs.imp ht

lemma nonempty_of_not_subset (h : ¬s ⊆ t) : (s \ t).nonempty :=
let ⟨x, xs, xt⟩ := not_subset.1 h in ⟨x, xs, xt⟩

lemma nonempty_of_ssubset (ht : s ⊂ t) : (t \ s).nonempty :=
nonempty_of_not_subset ht.2

lemma nonempty.of_diff (h : (s \ t).nonempty) : s.nonempty := h.imp $ λ _, and.left

lemma nonempty_of_ssubset' (ht : s ⊂ t) : t.nonempty := (nonempty_of_ssubset ht).of_diff

lemma nonempty.inl (hs : s.nonempty) : (s ∪ t).nonempty := hs.imp $ λ _, or.inl

lemma nonempty.inr (ht : t.nonempty) : (s ∪ t).nonempty := ht.imp $ λ _, or.inr

@[simp] lemma union_nonempty : (s ∪ t).nonempty ↔ s.nonempty ∨ t.nonempty := exists_or_distrib

lemma nonempty.left (h : (s ∩ t).nonempty) : s.nonempty := h.imp $ λ _, and.left

lemma nonempty.right (h : (s ∩ t).nonempty) : t.nonempty := h.imp $ λ _, and.right

lemma inter_nonempty : (s ∩ t).nonempty ↔ ∃ x, x ∈ s ∧ x ∈ t := iff.rfl

lemma inter_nonempty_iff_exists_left : (s ∩ t).nonempty ↔ ∃ x ∈ s, x ∈ t :=
by simp_rw [inter_nonempty, exists_prop]

lemma inter_nonempty_iff_exists_right : (s ∩ t).nonempty ↔ ∃ x ∈ t, x ∈ s :=
by simp_rw [inter_nonempty, exists_prop, and_comm]

lemma nonempty_iff_univ_nonempty : nonempty α ↔ (univ : set α).nonempty :=
⟨λ ⟨x⟩, ⟨x, trivial⟩, λ ⟨x, _⟩, ⟨x⟩⟩

@[simp] lemma univ_nonempty : ∀ [h : nonempty α], (univ : set α).nonempty
| ⟨x⟩ := ⟨x, trivial⟩

lemma nonempty.to_subtype : s.nonempty → nonempty s := nonempty_subtype.2
lemma nonempty.to_type : s.nonempty → nonempty α := λ ⟨x, hx⟩, ⟨x⟩

instance [nonempty α] : nonempty (set.univ : set α) := set.univ_nonempty.to_subtype

lemma nonempty_of_nonempty_subtype [nonempty s] : s.nonempty :=
nonempty_subtype.mp ‹_›

/-! ### Lemmas about the empty set -/

theorem empty_def : (∅ : set α) = {x | false} := rfl

@[simp] theorem mem_empty_iff_false (x : α) : x ∈ (∅ : set α) ↔ false := iff.rfl

@[simp] theorem set_of_false : {a : α | false} = ∅ := rfl

@[simp] theorem empty_subset (s : set α) : ∅ ⊆ s.

theorem subset_empty_iff {s : set α} : s ⊆ ∅ ↔ s = ∅ :=
(subset.antisymm_iff.trans $ and_iff_left (empty_subset _)).symm

theorem eq_empty_iff_forall_not_mem {s : set α} : s = ∅ ↔ ∀ x, x ∉ s := subset_empty_iff.symm

lemma eq_empty_of_forall_not_mem (h : ∀ x, x ∉ s) : s = ∅ := subset_empty_iff.1 h

theorem eq_empty_of_subset_empty {s : set α} : s ⊆ ∅ → s = ∅ := subset_empty_iff.1

theorem eq_empty_of_is_empty [is_empty α] (s : set α) : s = ∅ :=
eq_empty_of_subset_empty $ λ x hx, is_empty_elim x

/-- There is exactly one set of a type that is empty. -/
instance unique_empty [is_empty α] : unique (set α) :=
{ default := ∅, uniq := eq_empty_of_is_empty }

/-- See also `set.nonempty_iff_ne_empty`. -/
lemma not_nonempty_iff_eq_empty {s : set α} : ¬s.nonempty ↔ s = ∅ :=
by simp only [set.nonempty, eq_empty_iff_forall_not_mem, not_exists]

/-- See also `set.not_nonempty_iff_eq_empty`. -/
lemma nonempty_iff_ne_empty : s.nonempty ↔ s ≠ ∅ := not_nonempty_iff_eq_empty.not_right

alias nonempty_iff_ne_empty ↔ nonempty.ne_empty _

@[simp] lemma not_nonempty_empty : ¬(∅ : set α).nonempty := λ ⟨x, hx⟩, hx

@[simp] lemma is_empty_coe_sort {s : set α} : is_empty ↥s ↔ s = ∅ :=
not_iff_not.1 $ by simpa using nonempty_iff_ne_empty

lemma eq_empty_or_nonempty (s : set α) : s = ∅ ∨ s.nonempty :=
or_iff_not_imp_left.2 nonempty_iff_ne_empty.2

theorem subset_eq_empty {s t : set α} (h : t ⊆ s) (e : s = ∅) : t = ∅ :=
subset_empty_iff.1 $ e ▸ h

theorem ball_empty_iff {p : α → Prop} : (∀ x ∈ (∅ : set α), p x) ↔ true :=
iff_true_intro $ λ x, false.elim

instance (α : Type u) : is_empty.{u+1} (∅ : set α) :=
⟨λ x, x.2⟩

@[simp] lemma empty_ssubset : ∅ ⊂ s ↔ s.nonempty :=
(@bot_lt_iff_ne_bot (set α) _ _ _).trans nonempty_iff_ne_empty.symm

alias empty_ssubset ↔ _ nonempty.empty_ssubset

/-!

### Universal set.

In Lean `@univ α` (or `univ : set α`) is the set that contains all elements of type `α`.
Mathematically it is the same as `α` but it has a different type.

-/

@[simp] theorem set_of_true : {x : α | true} = univ := rfl

@[simp] theorem mem_univ (x : α) : x ∈ @univ α := trivial

@[simp] lemma univ_eq_empty_iff : (univ : set α) = ∅ ↔ is_empty α :=
eq_empty_iff_forall_not_mem.trans ⟨λ H, ⟨λ x, H x trivial⟩, λ H x _, @is_empty.false α H x⟩

theorem empty_ne_univ [nonempty α] : (∅ : set α) ≠ univ :=
λ e, not_is_empty_of_nonempty α $ univ_eq_empty_iff.1 e.symm

@[simp] theorem subset_univ (s : set α) : s ⊆ univ := λ x H, trivial

theorem univ_subset_iff {s : set α} : univ ⊆ s ↔ s = univ := @top_le_iff _ _ _ s

alias univ_subset_iff ↔ eq_univ_of_univ_subset _

theorem eq_univ_iff_forall {s : set α} : s = univ ↔ ∀ x, x ∈ s :=
univ_subset_iff.symm.trans $ forall_congr $ λ x, imp_iff_right trivial

theorem eq_univ_of_forall {s : set α} : (∀ x, x ∈ s) → s = univ := eq_univ_iff_forall.2

lemma nonempty.eq_univ [subsingleton α] : s.nonempty → s = univ :=
by { rintro ⟨x, hx⟩, refine eq_univ_of_forall (λ y, by rwa subsingleton.elim y x) }

lemma eq_univ_of_subset {s t : set α} (h : s ⊆ t) (hs : s = univ) : t = univ :=
eq_univ_of_univ_subset $ hs ▸ h

lemma exists_mem_of_nonempty (α) : ∀ [nonempty α], ∃x:α, x ∈ (univ : set α)
| ⟨x⟩ := ⟨x, trivial⟩

lemma ne_univ_iff_exists_not_mem {α : Type*} (s : set α) : s ≠ univ ↔ ∃ a, a ∉ s :=
by rw [←not_forall, ←eq_univ_iff_forall]

lemma not_subset_iff_exists_mem_not_mem {α : Type*} {s t : set α} :
  ¬ s ⊆ t ↔ ∃ x, x ∈ s ∧ x ∉ t :=
by simp [subset_def]

lemma univ_unique [unique α] : @set.univ α = {default} :=
set.ext $ λ x, iff_of_true trivial $ subsingleton.elim x default

lemma ssubset_univ_iff : s ⊂ univ ↔ s ≠ univ := lt_top_iff_ne_top

instance nontrivial_of_nonempty [nonempty α] : nontrivial (set α) := ⟨⟨∅, univ, empty_ne_univ⟩⟩

/-! ### Lemmas about union -/

theorem union_def {s₁ s₂ : set α} : s₁ ∪ s₂ = {a | a ∈ s₁ ∨ a ∈ s₂} := rfl

theorem mem_union_left {x : α} {a : set α} (b : set α) : x ∈ a → x ∈ a ∪ b := or.inl

theorem mem_union_right {x : α} {b : set α} (a : set α) : x ∈ b → x ∈ a ∪ b := or.inr

theorem mem_or_mem_of_mem_union {x : α} {a b : set α} (H : x ∈ a ∪ b) : x ∈ a ∨ x ∈ b := H

theorem mem_union.elim {x : α} {a b : set α} {P : Prop}
    (H₁ : x ∈ a ∪ b) (H₂ : x ∈ a → P) (H₃ : x ∈ b → P) : P :=
or.elim H₁ H₂ H₃

@[simp] theorem mem_union (x : α) (a b : set α) : x ∈ a ∪ b ↔ (x ∈ a ∨ x ∈ b) := iff.rfl

@[simp] theorem union_self (a : set α) : a ∪ a = a := ext $ λ x, or_self _

@[simp] theorem union_empty (a : set α) : a ∪ ∅ = a := ext $ λ x, or_false _

@[simp] theorem empty_union (a : set α) : ∅ ∪ a = a := ext $ λ x, false_or _

theorem union_comm (a b : set α) : a ∪ b = b ∪ a := ext $ λ x, or.comm

theorem union_assoc (a b c : set α) : (a ∪ b) ∪ c = a ∪ (b ∪ c) := ext $ λ x, or.assoc

instance union_is_assoc : is_associative (set α) (∪) := ⟨union_assoc⟩

instance union_is_comm : is_commutative (set α) (∪) := ⟨union_comm⟩

theorem union_left_comm (s₁ s₂ s₃ : set α) : s₁ ∪ (s₂ ∪ s₃) = s₂ ∪ (s₁ ∪ s₃) :=
ext $ λ x, or.left_comm

theorem union_right_comm (s₁ s₂ s₃ : set α) : (s₁ ∪ s₂) ∪ s₃ = (s₁ ∪ s₃) ∪ s₂ :=
ext $ λ x, or.right_comm

@[simp] theorem union_eq_left_iff_subset {s t : set α} : s ∪ t = s ↔ t ⊆ s :=
sup_eq_left

@[simp] theorem union_eq_right_iff_subset {s t : set α} : s ∪ t = t ↔ s ⊆ t :=
sup_eq_right

theorem union_eq_self_of_subset_left {s t : set α} (h : s ⊆ t) : s ∪ t = t :=
union_eq_right_iff_subset.mpr h

theorem union_eq_self_of_subset_right {s t : set α} (h : t ⊆ s) : s ∪ t = s :=
union_eq_left_iff_subset.mpr h

@[simp] theorem subset_union_left (s t : set α) : s ⊆ s ∪ t := λ x, or.inl

@[simp] theorem subset_union_right (s t : set α) : t ⊆ s ∪ t := λ x, or.inr

theorem union_subset {s t r : set α} (sr : s ⊆ r) (tr : t ⊆ r) : s ∪ t ⊆ r :=
λ x, or.rec (@sr _) (@tr _)

@[simp] theorem union_subset_iff {s t u : set α} : s ∪ t ⊆ u ↔ s ⊆ u ∧ t ⊆ u :=
(forall_congr (by exact λ x, or_imp_distrib)).trans forall_and_distrib

theorem union_subset_union {s₁ s₂ t₁ t₂ : set α}
  (h₁ : s₁ ⊆ s₂) (h₂ : t₁ ⊆ t₂) : s₁ ∪ t₁ ⊆ s₂ ∪ t₂ := λ x, or.imp (@h₁ _) (@h₂ _)

theorem union_subset_union_left {s₁ s₂ : set α} (t) (h : s₁ ⊆ s₂) : s₁ ∪ t ⊆ s₂ ∪ t :=
union_subset_union h subset.rfl

theorem union_subset_union_right (s) {t₁ t₂ : set α} (h : t₁ ⊆ t₂) : s ∪ t₁ ⊆ s ∪ t₂ :=
union_subset_union subset.rfl h

lemma subset_union_of_subset_left {s t : set α} (h : s ⊆ t) (u : set α) : s ⊆ t ∪ u :=
subset.trans h (subset_union_left t u)

lemma subset_union_of_subset_right {s u : set α} (h : s ⊆ u) (t : set α) : s ⊆ t ∪ u :=
subset.trans h (subset_union_right t u)

lemma union_congr_left (ht : t ⊆ s ∪ u) (hu : u ⊆ s ∪ t) : s ∪ t = s ⊔ u := sup_congr_left ht hu
lemma union_congr_right (hs : s ⊆ t ∪ u) (ht : t ⊆ s ∪ u) : s ∪ u = t ∪ u := sup_congr_right hs ht

lemma union_eq_union_iff_left : s ∪ t = s ∪ u ↔ t ⊆ s ∪ u ∧ u ⊆ s ∪ t := sup_eq_sup_iff_left
lemma union_eq_union_iff_right : s ∪ u = t ∪ u ↔ s ⊆ t ∪ u ∧ t ⊆ s ∪ u := sup_eq_sup_iff_right

@[simp] theorem union_empty_iff {s t : set α} : s ∪ t = ∅ ↔ s = ∅ ∧ t = ∅ :=
by simp only [← subset_empty_iff]; exact union_subset_iff

@[simp] lemma union_univ {s : set α} : s ∪ univ = univ := sup_top_eq

@[simp] lemma univ_union {s : set α} : univ ∪ s = univ := top_sup_eq

/-! ### Lemmas about intersection -/

theorem inter_def {s₁ s₂ : set α} : s₁ ∩ s₂ = {a | a ∈ s₁ ∧ a ∈ s₂} := rfl

@[simp] theorem mem_inter_iff (x : α) (a b : set α) : x ∈ a ∩ b ↔ (x ∈ a ∧ x ∈ b) := iff.rfl

theorem mem_inter {x : α} {a b : set α} (ha : x ∈ a) (hb : x ∈ b) : x ∈ a ∩ b := ⟨ha, hb⟩

theorem mem_of_mem_inter_left {x : α} {a b : set α} (h : x ∈ a ∩ b) : x ∈ a := h.left

theorem mem_of_mem_inter_right {x : α} {a b : set α} (h : x ∈ a ∩ b) : x ∈ b := h.right

@[simp] theorem inter_self (a : set α) : a ∩ a = a := ext $ λ x, and_self _

@[simp] theorem inter_empty (a : set α) : a ∩ ∅ = ∅ := ext $ λ x, and_false _

@[simp] theorem empty_inter (a : set α) : ∅ ∩ a = ∅ := ext $ λ x, false_and _

theorem inter_comm (a b : set α) : a ∩ b = b ∩ a := ext $ λ x, and.comm

theorem inter_assoc (a b c : set α) : (a ∩ b) ∩ c = a ∩ (b ∩ c) := ext $ λ x, and.assoc

instance inter_is_assoc : is_associative (set α) (∩) := ⟨inter_assoc⟩

instance inter_is_comm : is_commutative (set α) (∩) := ⟨inter_comm⟩

theorem inter_left_comm (s₁ s₂ s₃ : set α) : s₁ ∩ (s₂ ∩ s₃) = s₂ ∩ (s₁ ∩ s₃) :=
ext $ λ x, and.left_comm

theorem inter_right_comm (s₁ s₂ s₃ : set α) : (s₁ ∩ s₂) ∩ s₃ = (s₁ ∩ s₃) ∩ s₂ :=
ext $ λ x, and.right_comm

@[simp] theorem inter_subset_left (s t : set α) : s ∩ t ⊆ s := λ x, and.left

@[simp] theorem inter_subset_right (s t : set α) : s ∩ t ⊆ t := λ x, and.right

theorem subset_inter {s t r : set α} (rs : r ⊆ s) (rt : r ⊆ t) : r ⊆ s ∩ t := λ x h, ⟨rs h, rt h⟩

@[simp] theorem subset_inter_iff {s t r : set α} : r ⊆ s ∩ t ↔ r ⊆ s ∧ r ⊆ t :=
(forall_congr (by exact λ x, imp_and_distrib)).trans forall_and_distrib

@[simp] theorem inter_eq_left_iff_subset {s t : set α} : s ∩ t = s ↔ s ⊆ t :=
inf_eq_left

@[simp] theorem inter_eq_right_iff_subset {s t : set α} : s ∩ t = t ↔ t ⊆ s :=
inf_eq_right

theorem inter_eq_self_of_subset_left {s t : set α} : s ⊆ t → s ∩ t = s :=
inter_eq_left_iff_subset.mpr

theorem inter_eq_self_of_subset_right {s t : set α} : t ⊆ s → s ∩ t = t :=
inter_eq_right_iff_subset.mpr

lemma inter_congr_left (ht : s ∩ u ⊆ t) (hu : s ∩ t ⊆ u) : s ∩ t = s ∩ u := inf_congr_left ht hu
lemma inter_congr_right (hs : t ∩ u ⊆ s) (ht : s ∩ u ⊆ t) : s ∩ u = t ∩ u := inf_congr_right hs ht

lemma inter_eq_inter_iff_left : s ∩ t = s ∩ u ↔ s ∩ u ⊆ t ∧ s ∩ t ⊆ u := inf_eq_inf_iff_left
lemma inter_eq_inter_iff_right : s ∩ u = t ∩ u ↔ t ∩ u ⊆ s ∧ s ∩ u ⊆ t := inf_eq_inf_iff_right

@[simp] theorem inter_univ (a : set α) : a ∩ univ = a := inf_top_eq

@[simp] theorem univ_inter (a : set α) : univ ∩ a = a := top_inf_eq

theorem inter_subset_inter {s₁ s₂ t₁ t₂ : set α}
  (h₁ : s₁ ⊆ t₁) (h₂ : s₂ ⊆ t₂) : s₁ ∩ s₂ ⊆ t₁ ∩ t₂ := λ x, and.imp (@h₁ _) (@h₂ _)

theorem inter_subset_inter_left {s t : set α} (u : set α) (H : s ⊆ t) : s ∩ u ⊆ t ∩ u :=
inter_subset_inter H subset.rfl

theorem inter_subset_inter_right {s t : set α} (u : set α) (H : s ⊆ t) : u ∩ s ⊆ u ∩ t :=
inter_subset_inter subset.rfl H

theorem union_inter_cancel_left {s t : set α} : (s ∪ t) ∩ s = s :=
inter_eq_self_of_subset_right $ subset_union_left _ _

theorem union_inter_cancel_right {s t : set α} : (s ∪ t) ∩ t = t :=
inter_eq_self_of_subset_right $ subset_union_right _ _

/-! ### Distributivity laws -/

theorem inter_distrib_left (s t u : set α) : s ∩ (t ∪ u) = (s ∩ t) ∪ (s ∩ u) :=
inf_sup_left
theorem inter_union_distrib_left {s t u : set α} : s ∩ (t ∪ u) = (s ∩ t) ∪ (s ∩ u) :=
inf_sup_left

theorem inter_distrib_right (s t u : set α) : (s ∪ t) ∩ u = (s ∩ u) ∪ (t ∩ u) :=
inf_sup_right
theorem union_inter_distrib_right {s t u : set α} : (s ∪ t) ∩ u = (s ∩ u) ∪ (t ∩ u) :=
inf_sup_right

theorem union_distrib_left (s t u : set α) : s ∪ (t ∩ u) = (s ∪ t) ∩ (s ∪ u) :=
sup_inf_left
theorem union_inter_distrib_left {s t u : set α} : s ∪ (t ∩ u) = (s ∪ t) ∩ (s ∪ u) :=
sup_inf_left

theorem union_distrib_right (s t u : set α) : (s ∩ t) ∪ u = (s ∪ u) ∩ (t ∪ u) :=
sup_inf_right
theorem inter_union_distrib_right {s t u : set α} : (s ∩ t) ∪ u = (s ∪ u) ∩ (t ∪ u) :=
sup_inf_right

lemma union_union_distrib_left (s t u : set α) : s ∪ (t ∪ u) = (s ∪ t) ∪ (s ∪ u) :=
sup_sup_distrib_left _ _ _

lemma union_union_distrib_right (s t u : set α) : (s ∪ t) ∪ u = (s ∪ u) ∪ (t ∪ u) :=
sup_sup_distrib_right _ _ _

lemma inter_inter_distrib_left (s t u : set α) : s ∩ (t ∩ u) = (s ∩ t) ∩ (s ∩ u) :=
inf_inf_distrib_left _ _ _

lemma inter_inter_distrib_right (s t u : set α) : (s ∩ t) ∩ u = (s ∩ u) ∩ (t ∩ u) :=
inf_inf_distrib_right _ _ _

lemma union_union_union_comm (s t u v : set α) : (s ∪ t) ∪ (u ∪ v) = (s ∪ u) ∪ (t ∪ v) :=
sup_sup_sup_comm _ _ _ _

lemma inter_inter_inter_comm (s t u v : set α) : (s ∩ t) ∩ (u ∩ v) = (s ∩ u) ∩ (t ∩ v) :=
inf_inf_inf_comm _ _ _ _

/-!
### Lemmas about `insert`

`insert α s` is the set `{α} ∪ s`.
-/

theorem insert_def (x : α) (s : set α) : insert x s = { y | y = x ∨ y ∈ s } := rfl

@[simp] theorem subset_insert (x : α) (s : set α) : s ⊆ insert x s := λ y, or.inr

theorem mem_insert (x : α) (s : set α) : x ∈ insert x s := or.inl rfl

theorem mem_insert_of_mem {x : α} {s : set α} (y : α) : x ∈ s → x ∈ insert y s := or.inr

theorem eq_or_mem_of_mem_insert {x a : α} {s : set α} : x ∈ insert a s → x = a ∨ x ∈ s := id

lemma mem_of_mem_insert_of_ne : b ∈ insert a s → b ≠ a → b ∈ s := or.resolve_left
lemma eq_of_not_mem_of_mem_insert : b ∈ insert a s → b ∉ s → b = a := or.resolve_right

@[simp] theorem mem_insert_iff {x a : α} {s : set α} : x ∈ insert a s ↔ x = a ∨ x ∈ s := iff.rfl

@[simp] theorem insert_eq_of_mem {a : α} {s : set α} (h : a ∈ s) : insert a s = s :=
ext $ λ x, or_iff_right_of_imp $ λ e, e.symm ▸ h

lemma ne_insert_of_not_mem {s : set α} (t : set α) {a : α} : a ∉ s → s ≠ insert a t :=
mt $ λ e, e.symm ▸ mem_insert _ _

@[simp] lemma insert_eq_self : insert a s = s ↔ a ∈ s := ⟨λ h, h ▸ mem_insert _ _, insert_eq_of_mem⟩

lemma insert_ne_self : insert a s ≠ s ↔ a ∉ s := insert_eq_self.not

theorem insert_subset : insert a s ⊆ t ↔ (a ∈ t ∧ s ⊆ t) :=
by simp only [subset_def, or_imp_distrib, forall_and_distrib, forall_eq, mem_insert_iff]

theorem insert_subset_insert (h : s ⊆ t) : insert a s ⊆ insert a t := λ x, or.imp_right (@h _)

theorem insert_subset_insert_iff (ha : a ∉ s) : insert a s ⊆ insert a t ↔ s ⊆ t :=
begin
  refine ⟨λ h x hx, _, insert_subset_insert⟩,
  rcases h (subset_insert _ _ hx) with (rfl|hxt),
  exacts [(ha hx).elim, hxt]
end

theorem subset_insert_iff_of_not_mem {s t : set α} {a : α} (h : a ∉ s) :
  s ⊆ insert a t ↔ s ⊆ t :=
begin
  constructor,
  { intros g y hy,
    specialize g hy,
    rw [mem_insert_iff] at g,
    rcases g with g | g,
    { rw [g] at hy,
      contradiction },
    { assumption }},
  { intros g y hy,
    specialize g hy,
    exact mem_insert_of_mem _ g }
end

theorem ssubset_iff_insert {s t : set α} : s ⊂ t ↔ ∃ a ∉ s, insert a s ⊆ t :=
begin
  simp only [insert_subset, exists_and_distrib_right, ssubset_def, not_subset],
  simp only [exists_prop, and_comm]
end

theorem ssubset_insert {s : set α} {a : α} (h : a ∉ s) : s ⊂ insert a s :=
ssubset_iff_insert.2 ⟨a, h, subset.rfl⟩

theorem insert_comm (a b : α) (s : set α) : insert a (insert b s) = insert b (insert a s) :=
ext $ λ x, or.left_comm

@[simp] lemma insert_idem (a : α) (s : set α) : insert a (insert a s) = insert a s :=
insert_eq_of_mem $ mem_insert _ _

theorem insert_union : insert a s ∪ t = insert a (s ∪ t) := ext $ λ x, or.assoc

@[simp] theorem union_insert : s ∪ insert a t = insert a (s ∪ t) := ext $ λ x, or.left_comm

@[simp] theorem insert_nonempty (a : α) (s : set α) : (insert a s).nonempty := ⟨a, mem_insert a s⟩

instance (a : α) (s : set α) : nonempty (insert a s : set α) := (insert_nonempty a s).to_subtype

lemma insert_inter_distrib (a : α) (s t : set α) : insert a (s ∩ t) = insert a s ∩ insert a t :=
ext $ λ y, or_and_distrib_left

lemma insert_union_distrib (a : α) (s t : set α) : insert a (s ∪ t) = insert a s ∪ insert a t :=
ext $ λ _, or_or_distrib_left _ _ _

lemma insert_inj (ha : a ∉ s) : insert a s = insert b s ↔ a = b :=
⟨λ h, eq_of_not_mem_of_mem_insert (h.subst $ mem_insert a s) ha, congr_arg _⟩

-- useful in proofs by induction
theorem forall_of_forall_insert {P : α → Prop} {a : α} {s : set α}
  (H : ∀ x, x ∈ insert a s → P x) (x) (h : x ∈ s) : P x := H _ (or.inr h)

theorem forall_insert_of_forall {P : α → Prop} {a : α} {s : set α}
  (H : ∀ x, x ∈ s → P x) (ha : P a) (x) (h : x ∈ insert a s) : P x :=
h.elim (λ e, e.symm ▸ ha) (H _)

theorem bex_insert_iff {P : α → Prop} {a : α} {s : set α} :
  (∃ x ∈ insert a s, P x) ↔ P a ∨ (∃ x ∈ s, P x) :=
bex_or_left_distrib.trans $ or_congr_left' bex_eq_left

theorem ball_insert_iff {P : α → Prop} {a : α} {s : set α} :
  (∀ x ∈ insert a s, P x) ↔ P a ∧ (∀x ∈ s, P x) :=
ball_or_left_distrib.trans $ and_congr_left' forall_eq

/-! ### Lemmas about singletons -/

theorem singleton_def (a : α) : ({a} : set α) = insert a ∅ := (insert_emptyc_eq _).symm

@[simp] theorem mem_singleton_iff {a b : α} : a ∈ ({b} : set α) ↔ a = b := iff.rfl

@[simp] lemma set_of_eq_eq_singleton {a : α} : {n | n = a} = {a} := rfl

@[simp] lemma set_of_eq_eq_singleton' {a : α} : {x | a = x} = {a} := ext $ λ x, eq_comm

-- TODO: again, annotation needed
@[simp] theorem mem_singleton (a : α) : a ∈ ({a} : set α) := @rfl _ _

theorem eq_of_mem_singleton {x y : α} (h : x ∈ ({y} : set α)) : x = y := h

@[simp] theorem singleton_eq_singleton_iff {x y : α} : {x} = ({y} : set α) ↔ x = y :=
ext_iff.trans eq_iff_eq_cancel_left

lemma singleton_injective : injective (singleton : α → set α) :=
λ _ _, singleton_eq_singleton_iff.mp

theorem mem_singleton_of_eq {x y : α} (H : x = y) : x ∈ ({y} : set α) := H

theorem insert_eq (x : α) (s : set α) : insert x s = ({x} : set α) ∪ s := rfl

@[simp] theorem singleton_nonempty (a : α) : ({a} : set α).nonempty :=
⟨a, rfl⟩

@[simp] lemma singleton_ne_empty (a : α) : ({a} : set α) ≠ ∅ := (singleton_nonempty _).ne_empty

@[simp] lemma empty_ssubset_singleton : (∅ : set α) ⊂ {a} := (singleton_nonempty _).empty_ssubset

@[simp] theorem singleton_subset_iff {a : α} {s : set α} : {a} ⊆ s ↔ a ∈ s := forall_eq

theorem set_compr_eq_eq_singleton {a : α} : {b | b = a} = {a} := rfl

@[simp] theorem singleton_union : {a} ∪ s = insert a s := rfl

@[simp] theorem union_singleton : s ∪ {a} = insert a s := union_comm _ _

@[simp] theorem singleton_inter_nonempty : ({a} ∩ s).nonempty ↔ a ∈ s :=
by simp only [set.nonempty, mem_inter_iff, mem_singleton_iff, exists_eq_left]

@[simp] theorem inter_singleton_nonempty : (s ∩ {a}).nonempty ↔ a ∈ s :=
by rw [inter_comm, singleton_inter_nonempty]

@[simp] theorem singleton_inter_eq_empty : {a} ∩ s = ∅ ↔ a ∉ s :=
not_nonempty_iff_eq_empty.symm.trans singleton_inter_nonempty.not

@[simp] theorem inter_singleton_eq_empty : s ∩ {a} = ∅ ↔ a ∉ s :=
by rw [inter_comm, singleton_inter_eq_empty]

lemma nmem_singleton_empty {s : set α} : s ∉ ({∅} : set (set α)) ↔ s.nonempty :=
nonempty_iff_ne_empty.symm

instance unique_singleton (a : α) : unique ↥({a} : set α) :=
⟨⟨⟨a, mem_singleton a⟩⟩, λ ⟨x, h⟩, subtype.eq h⟩

lemma eq_singleton_iff_unique_mem : s = {a} ↔ a ∈ s ∧ ∀ x ∈ s, x = a :=
subset.antisymm_iff.trans $ and.comm.trans $ and_congr_left' singleton_subset_iff

lemma eq_singleton_iff_nonempty_unique_mem : s = {a} ↔ s.nonempty ∧ ∀ x ∈ s, x = a :=
eq_singleton_iff_unique_mem.trans $ and_congr_left $ λ H, ⟨λ h', ⟨_, h'⟩, λ ⟨x, h⟩, H x h ▸ h⟩

-- while `simp` is capable of proving this, it is not capable of turning the LHS into the RHS.
@[simp] lemma default_coe_singleton (x : α) : (default : ({x} : set α)) = ⟨x, rfl⟩ := rfl

/-! ### Lemmas about pairs -/

@[simp] theorem pair_eq_singleton (a : α) : ({a, a} : set α) = {a} := union_self _

theorem pair_comm (a b : α) : ({a, b} : set α) = {b, a} := union_comm _ _

lemma pair_eq_pair_iff {x y z w : α} :
  ({x, y} : set α) = {z, w} ↔ x = z ∧ y = w ∨ x = w ∧ y = z :=
begin
  simp only [set.subset.antisymm_iff, set.insert_subset, set.mem_insert_iff, set.mem_singleton_iff,
    set.singleton_subset_iff],
  split,
  { tauto! },
  { rintro (⟨rfl,rfl⟩|⟨rfl,rfl⟩); simp }
end

/-! ### Lemmas about sets defined as `{x ∈ s | p x}`. -/

section sep
variables {p q : α → Prop} {x : α}

theorem mem_sep (xs : x ∈ s) (px : p x) : x ∈ {x ∈ s | p x} := ⟨xs, px⟩

@[simp] theorem sep_mem_eq : {x ∈ s | x ∈ t} = s ∩ t := rfl

@[simp] theorem mem_sep_iff : x ∈ {x ∈ s | p x} ↔ x ∈ s ∧ p x := iff.rfl

theorem sep_ext_iff : {x ∈ s | p x} = {x ∈ s | q x} ↔ ∀ x ∈ s, (p x ↔ q x) :=
by simp_rw [ext_iff, mem_sep_iff, and.congr_right_iff]

theorem sep_eq_of_subset (h : s ⊆ t) : {x ∈ t | x ∈ s} = s :=
inter_eq_self_of_subset_right h

@[simp] theorem sep_subset (s : set α) (p : α → Prop) : {x ∈ s | p x} ⊆ s := λ x, and.left

@[simp] lemma sep_eq_self_iff_mem_true : {x ∈ s | p x} = s ↔ ∀ x ∈ s, p x :=
by simp_rw [ext_iff, mem_sep_iff, and_iff_left_iff_imp]

@[simp] lemma sep_eq_empty_iff_mem_false : {x ∈ s | p x} = ∅ ↔ ∀ x ∈ s, ¬ p x :=
by simp_rw [ext_iff, mem_sep_iff, mem_empty_iff_false, iff_false, not_and]

@[simp] lemma sep_true : {x ∈ s | true} = s := inter_univ s

@[simp] lemma sep_false : {x ∈ s | false} = ∅ := inter_empty s

@[simp] lemma sep_empty (p : α → Prop) : {x ∈ (∅ : set α) | p x} = ∅ := empty_inter p

@[simp] lemma sep_univ : {x ∈ (univ : set α) | p x} = {x | p x} := univ_inter p

@[simp] lemma sep_union : {x ∈ s ∪ t | p x} = {x ∈ s | p x} ∪ {x ∈ t | p x} :=
union_inter_distrib_right

@[simp] lemma sep_inter : {x ∈ s ∩ t | p x} = {x ∈ s | p x} ∩ {x ∈ t | p x} :=
inter_inter_distrib_right s t p

@[simp] lemma sep_and : {x ∈ s | p x ∧ q x} = {x ∈ s | p x} ∩ {x ∈ s | q x} :=
inter_inter_distrib_left s p q

@[simp] lemma sep_or : {x ∈ s | p x ∨ q x} = {x ∈ s | p x} ∪ {x ∈ s | q x} :=
inter_union_distrib_left

@[simp] lemma sep_set_of : {x ∈ {y | p y} | q x} = {x | p x ∧ q x} := rfl

end sep

@[simp] lemma subset_singleton_iff {α : Type*} {s : set α} {x : α} : s ⊆ {x} ↔ ∀ y ∈ s, y = x :=
iff.rfl

lemma subset_singleton_iff_eq {s : set α} {x : α} : s ⊆ {x} ↔ s = ∅ ∨ s = {x} :=
begin
  obtain (rfl | hs) := s.eq_empty_or_nonempty,
  use ⟨λ _, or.inl rfl, λ _, empty_subset _⟩,
  simp [eq_singleton_iff_nonempty_unique_mem, hs, hs.ne_empty],
end

lemma nonempty.subset_singleton_iff (h : s.nonempty) : s ⊆ {a} ↔ s = {a} :=
subset_singleton_iff_eq.trans $ or_iff_right h.ne_empty

lemma ssubset_singleton_iff {s : set α} {x : α} : s ⊂ {x} ↔ s = ∅ :=
begin
  rw [ssubset_iff_subset_ne, subset_singleton_iff_eq, or_and_distrib_right, and_not_self, or_false,
    and_iff_left_iff_imp],
  exact λ h, ne_of_eq_of_ne h (singleton_ne_empty _).symm,
end

lemma eq_empty_of_ssubset_singleton {s : set α} {x : α} (hs : s ⊂ {x}) : s = ∅ :=
ssubset_singleton_iff.1 hs

/-! ### Disjointness -/

protected theorem disjoint_iff : disjoint s t ↔ s ∩ t ⊆ ∅ := disjoint_iff_inf_le

theorem disjoint_iff_inter_eq_empty : disjoint s t ↔ s ∩ t = ∅ :=
disjoint_iff

lemma _root_.disjoint.inter_eq : disjoint s t → s ∩ t = ∅ := disjoint.eq_bot

lemma disjoint_left : disjoint s t ↔ ∀ ⦃a⦄, a ∈ s → a ∉ t :=
disjoint_iff_inf_le.trans $ forall_congr $ λ _, not_and
lemma disjoint_right : disjoint s t ↔ ∀ ⦃a⦄, a ∈ t → a ∉ s := by rw [disjoint.comm, disjoint_left]

lemma not_disjoint_iff : ¬disjoint s t ↔ ∃ x, x ∈ s ∧ x ∈ t :=
set.disjoint_iff.not.trans $ not_forall.trans $ exists_congr $ λ x, not_not

lemma not_disjoint_iff_nonempty_inter : ¬disjoint s t ↔ (s ∩ t).nonempty := not_disjoint_iff

alias not_disjoint_iff_nonempty_inter ↔ _ nonempty.not_disjoint

lemma disjoint_or_nonempty_inter (s t : set α) : disjoint s t ∨ (s ∩ t).nonempty :=
(em _).imp_right not_disjoint_iff_nonempty_inter.mp

lemma disjoint_iff_forall_ne : disjoint s t ↔ ∀ (x ∈ s) (y ∈ t), x ≠ y :=
by simp only [ne.def, disjoint_left, @imp_not_comm _ (_ = _), forall_eq']

lemma _root_.disjoint.ne_of_mem (h : disjoint s t) {x y} (hx : x ∈ s) (hy : y ∈ t) : x ≠ y :=
disjoint_iff_forall_ne.mp h x hx y hy

lemma disjoint_of_subset_left (hs : s₁ ⊆ s₂) (h : disjoint s₂ t) : disjoint s₁ t := h.mono_left hs
lemma disjoint_of_subset_right (ht : t₁ ⊆ t₂) (h : disjoint s t₂) : disjoint s t₁ := h.mono_right ht

lemma disjoint_of_subset (hs : s₁ ⊆ s₂) (ht : t₁ ⊆ t₂) (h : disjoint s₂ t₂) : disjoint s₁ t₁ :=
h.mono hs ht

@[simp] lemma disjoint_union_left : disjoint (s ∪ t) u ↔ disjoint s u ∧ disjoint t u :=
disjoint_sup_left

@[simp] lemma disjoint_union_right : disjoint s (t ∪ u) ↔ disjoint s t ∧ disjoint s u :=
disjoint_sup_right

@[simp] lemma disjoint_empty (s : set α) : disjoint s ∅ := disjoint_bot_right
@[simp] lemma empty_disjoint (s : set α) : disjoint ∅ s := disjoint_bot_left

@[simp] lemma univ_disjoint : disjoint univ s ↔ s = ∅ := top_disjoint
@[simp] lemma disjoint_univ : disjoint s univ ↔ s = ∅ := disjoint_top

lemma disjoint_sdiff_left : disjoint (t \ s) s := disjoint_sdiff_self_left
lemma disjoint_sdiff_right : disjoint s (t \ s) := disjoint_sdiff_self_right

@[simp] lemma disjoint_singleton_left : disjoint {a} s ↔ a ∉ s :=
by simp [set.disjoint_iff, subset_def]; exact iff.rfl

@[simp] lemma disjoint_singleton_right : disjoint s {a} ↔ a ∉ s :=
disjoint.comm.trans disjoint_singleton_left

@[simp] lemma disjoint_singleton : disjoint ({a} : set α) {b} ↔ a ≠ b :=
by rw [disjoint_singleton_left, mem_singleton_iff]

lemma subset_diff : s ⊆ t \ u ↔ s ⊆ t ∧ disjoint s u := le_iff_subset.symm.trans le_sdiff

/-! ### Lemmas about complement -/

lemma compl_def (s : set α) : sᶜ = {x | x ∉ s} := rfl

theorem mem_compl {s : set α} {x : α} (h : x ∉ s) : x ∈ sᶜ := h

lemma compl_set_of {α} (p : α → Prop) : {a | p a}ᶜ = { a | ¬ p a } := rfl

theorem not_mem_of_mem_compl {s : set α} {x : α} (h : x ∈ sᶜ) : x ∉ s := h

@[simp] theorem mem_compl_iff (s : set α) (x : α) : x ∈ sᶜ ↔ (x ∉ s) := iff.rfl

lemma not_mem_compl_iff {x : α} : x ∉ sᶜ ↔ x ∈ s := not_not

@[simp] theorem inter_compl_self (s : set α) : s ∩ sᶜ = ∅ := inf_compl_eq_bot

@[simp] theorem compl_inter_self (s : set α) : sᶜ ∩ s = ∅ := compl_inf_eq_bot

@[simp] theorem compl_empty : (∅ : set α)ᶜ = univ := compl_bot

@[simp] theorem compl_union (s t : set α) : (s ∪ t)ᶜ = sᶜ ∩ tᶜ := compl_sup

theorem compl_inter (s t : set α) : (s ∩ t)ᶜ = sᶜ ∪ tᶜ := compl_inf

@[simp] theorem compl_univ : (univ : set α)ᶜ = ∅ := compl_top

@[simp] lemma compl_empty_iff {s : set α} : sᶜ = ∅ ↔ s = univ := compl_eq_bot

@[simp] lemma compl_univ_iff {s : set α} : sᶜ = univ ↔ s = ∅ := compl_eq_top

lemma compl_ne_univ : sᶜ ≠ univ ↔ s.nonempty := compl_univ_iff.not.trans nonempty_iff_ne_empty.symm
lemma nonempty_compl : sᶜ.nonempty ↔ s ≠ univ := (ne_univ_iff_exists_not_mem s).symm

lemma mem_compl_singleton_iff {a x : α} : x ∈ ({a} : set α)ᶜ ↔ x ≠ a := iff.rfl

lemma compl_singleton_eq (a : α) : ({a} : set α)ᶜ = {x | x ≠ a} := rfl

@[simp] lemma compl_ne_eq_singleton (a : α) : ({x | x ≠ a} : set α)ᶜ = {a} := compl_compl _

theorem union_eq_compl_compl_inter_compl (s t : set α) : s ∪ t = (sᶜ ∩ tᶜ)ᶜ :=
ext $ λ x, or_iff_not_and_not

theorem inter_eq_compl_compl_union_compl (s t : set α) : s ∩ t = (sᶜ ∪ tᶜ)ᶜ :=
ext $ λ x, and_iff_not_or_not

@[simp] theorem union_compl_self (s : set α) : s ∪ sᶜ = univ := eq_univ_iff_forall.2 $ λ x, em _

@[simp] theorem compl_union_self (s : set α) : sᶜ ∪ s = univ := by rw [union_comm, union_compl_self]

lemma compl_subset_comm : sᶜ ⊆ t ↔ tᶜ ⊆ s := @compl_le_iff_compl_le _ s _ _
lemma subset_compl_comm : s ⊆ tᶜ ↔ t ⊆ sᶜ := @le_compl_iff_le_compl _ _ _ t

@[simp] lemma compl_subset_compl : sᶜ ⊆ tᶜ ↔ t ⊆ s := @compl_le_compl_iff_le (set α) _ _ _

lemma subset_compl_iff_disjoint_left : s ⊆ tᶜ ↔ disjoint t s :=
@le_compl_iff_disjoint_left (set α) _ _ _

lemma subset_compl_iff_disjoint_right : s ⊆ tᶜ ↔ disjoint s t :=
@le_compl_iff_disjoint_right (set α) _ _ _

lemma disjoint_compl_left_iff_subset : disjoint sᶜ t ↔ t ⊆ s := disjoint_compl_left_iff
lemma disjoint_compl_right_iff_subset : disjoint s tᶜ ↔ s ⊆ t := disjoint_compl_right_iff

alias subset_compl_iff_disjoint_right ↔ _ _root_.disjoint.subset_compl_right
alias subset_compl_iff_disjoint_left ↔ _ _root_.disjoint.subset_compl_left
alias disjoint_compl_left_iff_subset ↔ _ _root_.has_subset.subset.disjoint_compl_left
alias disjoint_compl_right_iff_subset ↔ _ _root_.has_subset.subset.disjoint_compl_right

theorem subset_union_compl_iff_inter_subset {s t u : set α} : s ⊆ t ∪ uᶜ ↔ s ∩ u ⊆ t :=
(@is_compl_compl _ u _).le_sup_right_iff_inf_left_le

theorem compl_subset_iff_union {s t : set α} : sᶜ ⊆ t ↔ s ∪ t = univ :=
iff.symm $ eq_univ_iff_forall.trans $ forall_congr $ λ a, or_iff_not_imp_left

@[simp] lemma subset_compl_singleton_iff {a : α} {s : set α} : s ⊆ {a}ᶜ ↔ a ∉ s :=
subset_compl_comm.trans singleton_subset_iff

theorem inter_subset (a b c : set α) : a ∩ b ⊆ c ↔ a ⊆ bᶜ ∪ c :=
forall_congr $ λ x, and_imp.trans $ imp_congr_right $ λ _, imp_iff_not_or

lemma inter_compl_nonempty_iff {s t : set α} : (s ∩ tᶜ).nonempty ↔ ¬ s ⊆ t :=
(not_subset.trans $ exists_congr $ by exact λ x, by simp [mem_compl]).symm

/-! ### Lemmas about set difference -/

theorem diff_eq (s t : set α) : s \ t = s ∩ tᶜ := rfl

@[simp] theorem mem_diff {s t : set α} (x : α) : x ∈ s \ t ↔ x ∈ s ∧ x ∉ t := iff.rfl

theorem mem_diff_of_mem {s t : set α} {x : α} (h1 : x ∈ s) (h2 : x ∉ t) : x ∈ s \ t :=
⟨h1, h2⟩

lemma not_mem_diff_of_mem {s t : set α} {x : α} (hx : x ∈ t) : x ∉ s \ t :=
λ h, h.2 hx

theorem mem_of_mem_diff {s t : set α} {x : α} (h : x ∈ s \ t) : x ∈ s :=
h.left

theorem not_mem_of_mem_diff {s t : set α} {x : α} (h : x ∈ s \ t) : x ∉ t :=
h.right

theorem diff_eq_compl_inter {s t : set α} : s \ t = tᶜ ∩ s :=
by rw [diff_eq, inter_comm]

theorem nonempty_diff {s t : set α} : (s \ t).nonempty ↔ ¬ (s ⊆ t) := inter_compl_nonempty_iff

theorem diff_subset (s t : set α) : s \ t ⊆ s := show s \ t ≤ s, from sdiff_le

theorem union_diff_cancel' {s t u : set α} (h₁ : s ⊆ t) (h₂ : t ⊆ u) : t ∪ (u \ s) = u :=
sup_sdiff_cancel' h₁ h₂

theorem union_diff_cancel {s t : set α} (h : s ⊆ t) : s ∪ (t \ s) = t :=
sup_sdiff_cancel_right h

theorem union_diff_cancel_left {s t : set α} (h : s ∩ t ⊆ ∅) : (s ∪ t) \ s = t :=
disjoint.sup_sdiff_cancel_left $ disjoint_iff_inf_le.2 h

theorem union_diff_cancel_right {s t : set α} (h : s ∩ t ⊆ ∅) : (s ∪ t) \ t = s :=
disjoint.sup_sdiff_cancel_right $ disjoint_iff_inf_le.2 h

@[simp] theorem union_diff_left {s t : set α} : (s ∪ t) \ s = t \ s :=
sup_sdiff_left_self

@[simp] theorem union_diff_right {s t : set α} : (s ∪ t) \ t = s \ t :=
sup_sdiff_right_self

theorem union_diff_distrib {s t u : set α} : (s ∪ t) \ u = s \ u ∪ t \ u :=
sup_sdiff

theorem inter_diff_assoc (a b c : set α) : (a ∩ b) \ c = a ∩ (b \ c) :=
inf_sdiff_assoc

@[simp] theorem inter_diff_self (a b : set α) : a ∩ (b \ a) = ∅ :=
inf_sdiff_self_right

@[simp] theorem inter_union_diff (s t : set α) : (s ∩ t) ∪ (s \ t) = s :=
sup_inf_sdiff s t

@[simp] lemma diff_union_inter (s t : set α) : (s \ t) ∪ (s ∩ t) = s :=
by { rw union_comm, exact sup_inf_sdiff _ _ }

@[simp] theorem inter_union_compl (s t : set α) : (s ∩ t) ∪ (s ∩ tᶜ) = s := inter_union_diff _ _

theorem diff_subset_diff {s₁ s₂ t₁ t₂ : set α} : s₁ ⊆ s₂ → t₂ ⊆ t₁ → s₁ \ t₁ ⊆ s₂ \ t₂ :=
show s₁ ≤ s₂ → t₂ ≤ t₁ → s₁ \ t₁ ≤ s₂ \ t₂, from sdiff_le_sdiff

theorem diff_subset_diff_left {s₁ s₂ t : set α} (h : s₁ ⊆ s₂) : s₁ \ t ⊆ s₂ \ t :=
sdiff_le_sdiff_right ‹s₁ ≤ s₂›

theorem diff_subset_diff_right {s t u : set α} (h : t ⊆ u) : s \ u ⊆ s \ t :=
sdiff_le_sdiff_left ‹t ≤ u›

theorem compl_eq_univ_diff (s : set α) : sᶜ = univ \ s :=
top_sdiff.symm

@[simp] lemma empty_diff (s : set α) : (∅ \ s : set α) = ∅ :=
bot_sdiff

theorem diff_eq_empty {s t : set α} : s \ t = ∅ ↔ s ⊆ t :=
sdiff_eq_bot_iff

@[simp] theorem diff_empty {s : set α} : s \ ∅ = s :=
sdiff_bot

@[simp] lemma diff_univ (s : set α) : s \ univ = ∅ := diff_eq_empty.2 (subset_univ s)

theorem diff_diff {u : set α} : s \ t \ u = s \ (t ∪ u) :=
sdiff_sdiff_left

-- the following statement contains parentheses to help the reader
lemma diff_diff_comm {s t u : set α} : (s \ t) \ u = (s \ u) \ t :=
sdiff_sdiff_comm

lemma diff_subset_iff {s t u : set α} : s \ t ⊆ u ↔ s ⊆ t ∪ u :=
show s \ t ≤ u ↔ s ≤ t ∪ u, from sdiff_le_iff

lemma subset_diff_union (s t : set α) : s ⊆ (s \ t) ∪ t :=
show s ≤ (s \ t) ∪ t, from le_sdiff_sup

lemma diff_union_of_subset {s t : set α} (h : t ⊆ s) :
  (s \ t) ∪ t = s :=
subset.antisymm (union_subset (diff_subset _ _) h) (subset_diff_union _ _)

@[simp] lemma diff_singleton_subset_iff {x : α} {s t : set α} : s \ {x} ⊆ t ↔ s ⊆ insert x t :=
by { rw [←union_singleton, union_comm], apply diff_subset_iff }

lemma subset_diff_singleton {x : α} {s t : set α} (h : s ⊆ t) (hx : x ∉ s) : s ⊆ t \ {x} :=
subset_inter h $ subset_compl_comm.1 $ singleton_subset_iff.2 hx

lemma subset_insert_diff_singleton (x : α) (s : set α) : s ⊆ insert x (s \ {x}) :=
by rw [←diff_singleton_subset_iff]

lemma diff_subset_comm {s t u : set α} : s \ t ⊆ u ↔ s \ u ⊆ t :=
show s \ t ≤ u ↔ s \ u ≤ t, from sdiff_le_comm

lemma diff_inter {s t u : set α} : s \ (t ∩ u) = (s \ t) ∪ (s \ u) :=
sdiff_inf

lemma diff_inter_diff {s t u : set α} : s \ t ∩ (s \ u) = s \ (t ∪ u) :=
sdiff_sup.symm

lemma diff_compl : s \ tᶜ = s ∩ t := sdiff_compl

lemma diff_diff_right {s t u : set α} : s \ (t \ u) = (s \ t) ∪ (s ∩ u) :=
sdiff_sdiff_right'

@[simp] theorem insert_diff_of_mem (s) (h : a ∈ t) : insert a s \ t = s \ t :=
by { ext, split; simp [or_imp_distrib, h] {contextual := tt} }

theorem insert_diff_of_not_mem (s) (h : a ∉ t) : insert a s \ t = insert a (s \ t) :=
begin
  classical,
  ext x,
  by_cases h' : x ∈ t,
  { have : x ≠ a,
    { assume H,
      rw H at h',
      exact h h' },
    simp [h, h', this] },
  { simp [h, h'] }
end

lemma insert_diff_self_of_not_mem {a : α} {s : set α} (h : a ∉ s) :
  insert a s \ {a} = s :=
by { ext, simp [and_iff_left_of_imp (λ hx : x ∈ s, show x ≠ a, from λ hxa, h $ hxa ▸ hx)] }

@[simp] lemma insert_diff_eq_singleton {a : α} {s : set α} (h : a ∉ s) :
  insert a s \ s = {a} :=
begin
  ext,
  rw [set.mem_diff, set.mem_insert_iff, set.mem_singleton_iff, or_and_distrib_right,
    and_not_self, or_false, and_iff_left_iff_imp],
  rintro rfl,
  exact h,
end

lemma inter_insert_of_mem (h : a ∈ s) : s ∩ insert a t = insert a (s ∩ t) :=
by rw [insert_inter_distrib, insert_eq_of_mem h]

lemma insert_inter_of_mem (h : a ∈ t) : insert a s ∩ t = insert a (s ∩ t) :=
by rw [insert_inter_distrib, insert_eq_of_mem h]

lemma inter_insert_of_not_mem (h : a ∉ s) : s ∩ insert a t = s ∩ t :=
ext $ λ x, and_congr_right $ λ hx, or_iff_right $ ne_of_mem_of_not_mem hx h

lemma insert_inter_of_not_mem (h : a ∉ t) : insert a s ∩ t = s ∩ t :=
ext $ λ x, and_congr_left $ λ hx, or_iff_right $ ne_of_mem_of_not_mem hx h

@[simp] lemma union_diff_self {s t : set α} : s ∪ (t \ s) = s ∪ t := sup_sdiff_self _ _
@[simp] lemma diff_union_self {s t : set α} : (s \ t) ∪ t = s ∪ t := sdiff_sup_self _ _

@[simp] theorem diff_inter_self {a b : set α} : (b \ a) ∩ a = ∅ :=
inf_sdiff_self_left

@[simp] theorem diff_inter_self_eq_diff {s t : set α} : s \ (t ∩ s) = s \ t :=
sdiff_inf_self_right _ _

@[simp] theorem diff_self_inter {s t : set α} : s \ (s ∩ t) = s \ t := sdiff_inf_self_left _ _

@[simp] theorem diff_singleton_eq_self {a : α} {s : set α} (h : a ∉ s) : s \ {a} = s :=
sdiff_eq_self_iff_disjoint.2 $ by simp [h]

@[simp] theorem insert_diff_singleton {a : α} {s : set α} :
  insert a (s \ {a}) = insert a s :=
by simp [insert_eq, union_diff_self, -union_singleton, -singleton_union]

@[simp] lemma diff_self {s : set α} : s \ s = ∅ := sdiff_self

lemma diff_diff_right_self (s t : set α)  : s \ (s \ t) = s ∩ t := sdiff_sdiff_right_self

lemma diff_diff_cancel_left {s t : set α} (h : s ⊆ t) : t \ (t \ s) = s :=
sdiff_sdiff_eq_self h

lemma mem_diff_singleton {x y : α} {s : set α} : x ∈ s \ {y} ↔ (x ∈ s ∧ x ≠ y) :=
iff.rfl

lemma mem_diff_singleton_empty {t : set (set α)} : s ∈ t \ {∅} ↔ s ∈ t ∧ s.nonempty :=
mem_diff_singleton.trans $ and_congr_right' nonempty_iff_ne_empty.symm

lemma union_eq_diff_union_diff_union_inter (s t : set α) :
  s ∪ t = (s \ t) ∪ (t \ s) ∪ (s ∩ t) :=
sup_eq_sdiff_sup_sdiff_sup_inf

/-! ### Symmetric difference -/

lemma mem_symm_diff : a ∈ s ∆ t ↔ a ∈ s ∧ a ∉ t ∨ a ∈ t ∧ a ∉ s := iff.rfl

protected lemma symm_diff_def (s t : set α) : s ∆ t = s \ t ∪ t \ s := rfl

lemma symm_diff_subset_union : s ∆ t ⊆ s ∪ t := @symm_diff_le_sup (set α) _ _ _

@[simp] lemma symm_diff_eq_empty : s ∆ t = ∅ ↔ s = t := symm_diff_eq_bot

@[simp] lemma symm_diff_nonempty : (s ∆ t).nonempty ↔ s ≠ t :=
nonempty_iff_ne_empty.trans symm_diff_eq_empty.not

lemma inter_symm_diff_distrib_left (s t u : set α) : s ∩ t ∆ u = (s ∩ t) ∆ (s ∩ u) :=
inf_symm_diff_distrib_left _ _ _

lemma inter_symm_diff_distrib_right (s t u : set α) : s ∆ t ∩ u = (s ∩ u) ∆ (t ∩ u) :=
inf_symm_diff_distrib_right _ _ _

lemma subset_symm_diff_union_symm_diff_left (h : disjoint s t) : u ⊆ s ∆ u ∪ t ∆ u :=
h.le_symm_diff_sup_symm_diff_left

lemma subset_symm_diff_union_symm_diff_right (h : disjoint t u) : s ⊆ s ∆ t ∪ s ∆ u :=
h.le_symm_diff_sup_symm_diff_right

/-! ### Powerset -/

/-- `𝒫 s = set.powerset s` is the set of all subsets of `s`. -/
def powerset (s : set α) : set (set α) := {t | t ⊆ s}

prefix `𝒫`:100 := powerset

theorem mem_powerset {x s : set α} (h : x ⊆ s) : x ∈ 𝒫 s := h

theorem subset_of_mem_powerset {x s : set α} (h : x ∈ 𝒫 s) : x ⊆ s := h

@[simp] theorem mem_powerset_iff (x s : set α) : x ∈ 𝒫 s ↔ x ⊆ s := iff.rfl

theorem powerset_inter (s t : set α) : 𝒫 (s ∩ t) = 𝒫 s ∩ 𝒫 t :=
ext $ λ u, subset_inter_iff

@[simp] theorem powerset_mono : 𝒫 s ⊆ 𝒫 t ↔ s ⊆ t :=
⟨λ h, h (subset.refl s), λ h u hu, subset.trans hu h⟩

theorem monotone_powerset : monotone (powerset : set α → set (set α)) :=
λ s t, powerset_mono.2

@[simp] theorem powerset_nonempty : (𝒫 s).nonempty :=
⟨∅, empty_subset s⟩

@[simp] theorem powerset_empty : 𝒫 (∅ : set α) = {∅} :=
ext $ λ s, subset_empty_iff

@[simp] theorem powerset_univ : 𝒫 (univ : set α) = univ :=
eq_univ_of_forall subset_univ

/- The powerset of a singleton contains only `∅` and the singleton itself. -/
theorem powerset_singleton (x : α) : 𝒫 ({x} : set α) = {∅, {x}} :=
by { ext y, rw [mem_powerset_iff, subset_singleton_iff_eq, mem_insert_iff, mem_singleton_iff] }

/-! ### Sets defined as an if-then-else -/

lemma mem_dite_univ_right (p : Prop) [decidable p] (t : p → set α) (x : α) :
  (x ∈ if h : p then t h else univ) ↔ (∀ h : p, x ∈ t h) :=
by split_ifs; simp [h]

@[simp] lemma mem_ite_univ_right (p : Prop) [decidable p] (t : set α) (x : α) :
  x ∈ ite p t set.univ ↔ (p → x ∈ t) :=
mem_dite_univ_right p (λ _, t) x

lemma mem_dite_univ_left (p : Prop) [decidable p] (t : ¬ p → set α) (x : α) :
  (x ∈ if h : p then univ else t h) ↔ (∀ h : ¬ p, x ∈ t h)  :=
by split_ifs; simp [h]

@[simp] lemma mem_ite_univ_left (p : Prop) [decidable p] (t : set α) (x : α) :
  x ∈ ite p set.univ t ↔ (¬ p → x ∈ t) :=
mem_dite_univ_left p (λ _, t) x

lemma mem_dite_empty_right (p : Prop) [decidable p] (t : p → set α) (x : α) :
  (x ∈ if h : p then t h else ∅) ↔ (∃ h : p, x ∈ t h) :=
by split_ifs; simp [h]

@[simp] lemma mem_ite_empty_right (p : Prop) [decidable p] (t : set α) (x : α) :
  x ∈ ite p t ∅ ↔ p ∧ x ∈ t :=
by split_ifs; simp [h]

lemma mem_dite_empty_left (p : Prop) [decidable p] (t : ¬ p → set α) (x : α) :
  (x ∈ if h : p then ∅ else t h) ↔ (∃ h : ¬ p, x ∈ t h) :=
by split_ifs; simp [h]

@[simp] lemma mem_ite_empty_left (p : Prop) [decidable p] (t : set α) (x : α) :
  x ∈ ite p ∅ t ↔ ¬ p ∧ x ∈ t :=
by split_ifs; simp [h]

/-! ### If-then-else for sets -/

/-- `ite` for sets: `set.ite t s s' ∩ t = s ∩ t`, `set.ite t s s' ∩ tᶜ = s' ∩ tᶜ`.
Defined as `s ∩ t ∪ s' \ t`. -/
protected def ite (t s s' : set α) : set α := s ∩ t ∪ s' \ t

@[simp] lemma ite_inter_self (t s s' : set α) : t.ite s s' ∩ t = s ∩ t :=
by rw [set.ite, union_inter_distrib_right, diff_inter_self, inter_assoc, inter_self, union_empty]

@[simp] lemma ite_compl (t s s' : set α) : tᶜ.ite s s' = t.ite s' s :=
by rw [set.ite, set.ite, diff_compl, union_comm, diff_eq]

@[simp] lemma ite_inter_compl_self (t s s' : set α) : t.ite s s' ∩ tᶜ = s' ∩ tᶜ :=
by rw [← ite_compl, ite_inter_self]

@[simp] lemma ite_diff_self (t s s' : set α) : t.ite s s' \ t = s' \ t :=
ite_inter_compl_self t s s'

@[simp] lemma ite_same (t s : set α) : t.ite s s = s := inter_union_diff _ _

@[simp] lemma ite_left (s t : set α) : s.ite s t = s ∪ t := by simp [set.ite]

@[simp] lemma ite_right (s t : set α) : s.ite t s = t ∩ s := by simp [set.ite]

@[simp] lemma ite_empty (s s' : set α) : set.ite ∅ s s' = s' :=
by simp [set.ite]

@[simp] lemma ite_univ (s s' : set α) : set.ite univ s s' = s :=
by simp [set.ite]

@[simp] lemma ite_empty_left (t s : set α) : t.ite ∅ s = s \ t :=
by simp [set.ite]

@[simp] lemma ite_empty_right (t s : set α) : t.ite s ∅ = s ∩ t :=
by simp [set.ite]

lemma ite_mono (t : set α) {s₁ s₁' s₂ s₂' : set α} (h : s₁ ⊆ s₂) (h' : s₁' ⊆ s₂') :
  t.ite s₁ s₁' ⊆ t.ite s₂ s₂' :=
union_subset_union (inter_subset_inter_left _ h) (inter_subset_inter_left _ h')

lemma ite_subset_union (t s s' : set α) : t.ite s s' ⊆ s ∪ s' :=
union_subset_union (inter_subset_left _ _) (diff_subset _ _)

lemma inter_subset_ite (t s s' : set α) : s ∩ s' ⊆ t.ite s s' :=
ite_same t (s ∩ s') ▸ ite_mono _ (inter_subset_left _ _) (inter_subset_right _ _)

lemma ite_inter_inter (t s₁ s₂ s₁' s₂' : set α) :
  t.ite (s₁ ∩ s₂) (s₁' ∩ s₂') = t.ite s₁ s₁' ∩ t.ite s₂ s₂' :=
by { ext x, simp only [set.ite, set.mem_inter_iff, set.mem_diff, set.mem_union], itauto }

lemma ite_inter (t s₁ s₂ s : set α) :
  t.ite (s₁ ∩ s) (s₂ ∩ s) = t.ite s₁ s₂ ∩ s :=
by rw [ite_inter_inter, ite_same]

lemma ite_inter_of_inter_eq (t : set α) {s₁ s₂ s : set α} (h : s₁ ∩ s = s₂ ∩ s) :
  t.ite s₁ s₂ ∩ s = s₁ ∩ s :=
by rw [← ite_inter, ← h, ite_same]

lemma subset_ite {t s s' u : set α} : u ⊆ t.ite s s' ↔ u ∩ t ⊆ s ∧ u \ t ⊆ s' :=
begin
  simp only [subset_def, ← forall_and_distrib],
  refine forall_congr (λ x, _),
  by_cases hx : x ∈ t; simp [*, set.ite]
end

/-! ### Subsingleton -/

/-- A set `s` is a `subsingleton` if it has at most one element. -/
protected def subsingleton (s : set α) : Prop :=
∀ ⦃x⦄ (hx : x ∈ s) ⦃y⦄ (hy : y ∈ s), x = y

lemma subsingleton.anti (ht : t.subsingleton) (hst : s ⊆ t) : s.subsingleton :=
λ x hx y hy, ht (hst hx) (hst hy)

lemma subsingleton.eq_singleton_of_mem (hs : s.subsingleton) {x:α} (hx : x ∈ s) : s = {x} :=
ext $ λ y, ⟨λ hy, (hs hx hy) ▸ mem_singleton _, λ hy, (eq_of_mem_singleton hy).symm ▸ hx⟩

@[simp] lemma subsingleton_empty : (∅ : set α).subsingleton := λ x, false.elim

@[simp] lemma subsingleton_singleton {a} : ({a} : set α).subsingleton :=
λ x hx y hy, (eq_of_mem_singleton hx).symm ▸ (eq_of_mem_singleton hy).symm ▸ rfl

lemma subsingleton_of_subset_singleton (h : s ⊆ {a}) : s.subsingleton :=
subsingleton_singleton.anti h

lemma subsingleton_of_forall_eq (a : α) (h : ∀ b ∈ s, b = a) : s.subsingleton :=
λ b hb c hc, (h _ hb).trans (h _ hc).symm

lemma subsingleton_iff_singleton {x} (hx : x ∈ s) : s.subsingleton ↔ s = {x} :=
⟨λ h, h.eq_singleton_of_mem hx, λ h,h.symm ▸ subsingleton_singleton⟩

lemma subsingleton.eq_empty_or_singleton (hs : s.subsingleton) :
  s = ∅ ∨ ∃ x, s = {x} :=
s.eq_empty_or_nonempty.elim or.inl (λ ⟨x, hx⟩, or.inr ⟨x, hs.eq_singleton_of_mem hx⟩)

lemma subsingleton.induction_on {p : set α → Prop} (hs : s.subsingleton) (he : p ∅)
  (h₁ : ∀ x, p {x}) : p s :=
by { rcases hs.eq_empty_or_singleton with rfl|⟨x, rfl⟩, exacts [he, h₁ _] }

lemma subsingleton_univ [subsingleton α] : (univ : set α).subsingleton :=
λ x hx y hy, subsingleton.elim x y

lemma subsingleton_of_univ_subsingleton (h : (univ : set α).subsingleton) : subsingleton α :=
⟨λ a b, h (mem_univ a) (mem_univ b)⟩

@[simp] lemma subsingleton_univ_iff : (univ : set α).subsingleton ↔ subsingleton α :=
⟨subsingleton_of_univ_subsingleton, λ h, @subsingleton_univ _ h⟩

lemma subsingleton_of_subsingleton [subsingleton α] {s : set α} : set.subsingleton s :=
subsingleton_univ.anti (subset_univ s)

lemma subsingleton_is_top (α : Type*) [partial_order α] : set.subsingleton {x : α | is_top x} :=
λ x hx y hy, hx.is_max.eq_of_le (hy x)

lemma subsingleton_is_bot (α : Type*) [partial_order α] : set.subsingleton {x : α | is_bot x} :=
λ x hx y hy, hx.is_min.eq_of_ge (hy x)

lemma exists_eq_singleton_iff_nonempty_subsingleton :
  (∃ a : α, s = {a}) ↔ s.nonempty ∧ s.subsingleton :=
begin
  refine ⟨_, λ h, _⟩,
  { rintros ⟨a, rfl⟩,
    exact ⟨singleton_nonempty a, subsingleton_singleton⟩ },
  { exact h.2.eq_empty_or_singleton.resolve_left h.1.ne_empty },
end

/-- `s`, coerced to a type, is a subsingleton type if and only if `s` is a subsingleton set. -/
@[simp, norm_cast] lemma subsingleton_coe (s : set α) : subsingleton s ↔ s.subsingleton :=
begin
  split,
  { refine λ h, (λ a ha b hb, _),
    exact set_coe.ext_iff.2 (@subsingleton.elim s h ⟨a, ha⟩ ⟨b, hb⟩) },
  { exact λ h, subsingleton.intro (λ a b, set_coe.ext (h a.property b.property)) }
end

lemma subsingleton.coe_sort {s : set α} : s.subsingleton → subsingleton s := s.subsingleton_coe.2

/-- The `coe_sort` of a set `s` in a subsingleton type is a subsingleton.
For the corresponding result for `subtype`, see `subtype.subsingleton`. -/
instance subsingleton_coe_of_subsingleton [subsingleton α] {s : set α} : subsingleton s :=
by { rw [s.subsingleton_coe], exact subsingleton_of_subsingleton }

/-! ### Nontrivial -/

/-- A set `s` is `nontrivial` if it has at least two distinct elements. -/
protected def nontrivial (s : set α) : Prop := ∃ x y ∈ s, x ≠ y

lemma nontrivial_of_mem_mem_ne {x y} (hx : x ∈ s) (hy : y ∈ s) (hxy : x ≠ y) : s.nontrivial :=
⟨x, hx, y, hy, hxy⟩

/-- Extract witnesses from s.nontrivial. This function might be used instead of case analysis on the
argument. Note that it makes a proof depend on the classical.choice axiom. -/
protected noncomputable def nontrivial.some (hs : s.nontrivial) : α × α :=
(hs.some, hs.some_spec.some_spec.some)

protected lemma nontrivial.some_fst_mem (hs : s.nontrivial) : hs.some.fst ∈ s := hs.some_spec.some

protected lemma nontrivial.some_snd_mem (hs : s.nontrivial) : hs.some.snd ∈ s :=
hs.some_spec.some_spec.some_spec.some

protected lemma nontrivial.some_fst_ne_some_snd (hs : s.nontrivial) : hs.some.fst ≠ hs.some.snd :=
hs.some_spec.some_spec.some_spec.some_spec

lemma nontrivial.mono (hs : s.nontrivial) (hst : s ⊆ t) : t.nontrivial :=
let ⟨x, hx, y, hy, hxy⟩ := hs in ⟨x, hst hx, y, hst hy, hxy⟩

lemma nontrivial_pair {x y} (hxy : x ≠ y) : ({x, y} : set α).nontrivial :=
⟨x, mem_insert _ _, y, mem_insert_of_mem _ (mem_singleton _), hxy⟩

lemma nontrivial_of_pair_subset {x y} (hxy : x ≠ y) (h : {x, y} ⊆ s) : s.nontrivial :=
(nontrivial_pair hxy).mono h

lemma nontrivial.pair_subset (hs : s.nontrivial) : ∃ x y (hab : x ≠ y), {x, y} ⊆ s :=
let ⟨x, hx, y, hy, hxy⟩ := hs in ⟨x, y, hxy, insert_subset.2 ⟨hx, (singleton_subset_iff.2 hy)⟩⟩

lemma nontrivial_iff_pair_subset : s.nontrivial ↔ ∃ x y (hxy : x ≠ y), {x, y} ⊆ s :=
⟨nontrivial.pair_subset, λ H, let ⟨x, y, hxy, h⟩ := H in nontrivial_of_pair_subset hxy h⟩

lemma nontrivial_of_exists_ne {x} (hx : x ∈ s) (h : ∃ y ∈ s, y ≠ x) : s.nontrivial :=
let ⟨y, hy, hyx⟩ := h in ⟨y, hy, x, hx, hyx⟩

lemma nontrivial.exists_ne (hs : s.nontrivial) (z) : ∃ x ∈ s, x ≠ z :=
begin
  by_contra H, push_neg at H,
  rcases hs with ⟨x, hx, y, hy, hxy⟩,
  rw [H x hx, H y hy] at hxy,
  exact hxy rfl
end

lemma nontrivial_iff_exists_ne {x} (hx : x ∈ s) : s.nontrivial ↔ ∃ y ∈ s, y ≠ x :=
⟨λ H, H.exists_ne _, nontrivial_of_exists_ne hx⟩

lemma nontrivial_of_lt [preorder α] {x y} (hx : x ∈ s) (hy : y ∈ s) (hxy : x < y) : s.nontrivial :=
⟨x, hx, y, hy, ne_of_lt hxy⟩

lemma nontrivial_of_exists_lt [preorder α] (H : ∃ x y ∈ s, x < y) : s.nontrivial :=
let ⟨x, hx, y, hy, hxy⟩ := H in nontrivial_of_lt hx hy hxy

lemma nontrivial.exists_lt [linear_order α] (hs : s.nontrivial) : ∃ x y ∈ s, x < y :=
let ⟨x, hx, y, hy, hxy⟩ := hs in
or.elim (lt_or_gt_of_ne hxy) (λ H, ⟨x, hx, y, hy, H⟩) (λ H, ⟨y, hy, x, hx, H⟩)

lemma nontrivial_iff_exists_lt [linear_order α] : s.nontrivial ↔ ∃ x y ∈ s, x < y :=
⟨nontrivial.exists_lt, nontrivial_of_exists_lt⟩

protected lemma nontrivial.nonempty (hs : s.nontrivial) : s.nonempty :=
let ⟨x, hx, _⟩ := hs in ⟨x, hx⟩

protected lemma nontrivial.ne_empty (hs : s.nontrivial) : s ≠ ∅ := hs.nonempty.ne_empty

lemma nontrivial.not_subset_empty (hs : s.nontrivial) : ¬ s ⊆ ∅ := hs.nonempty.not_subset_empty

@[simp] lemma not_nontrivial_empty : ¬ (∅ : set α).nontrivial := λ h, h.ne_empty rfl

@[simp] lemma not_nontrivial_singleton {x} : ¬ ({x} : set α).nontrivial :=
λ H, begin
  rw nontrivial_iff_exists_ne (mem_singleton x) at H,
  exact let ⟨y, hy, hya⟩ := H in hya (mem_singleton_iff.1 hy)
end

lemma nontrivial.ne_singleton {x} (hs : s.nontrivial) : s ≠ {x} :=
λ H, by { rw H at hs, exact not_nontrivial_singleton hs }

lemma nontrivial.not_subset_singleton {x} (hs : s.nontrivial) : ¬ s ⊆ {x} :=
(not_congr subset_singleton_iff_eq).2 (not_or hs.ne_empty hs.ne_singleton)

lemma nontrivial_univ [nontrivial α] : (univ : set α).nontrivial :=
let ⟨x, y, hxy⟩ := exists_pair_ne α in ⟨x, mem_univ _, y, mem_univ _, hxy⟩

lemma nontrivial_of_univ_nontrivial (h : (univ : set α).nontrivial) : nontrivial α :=
let ⟨x, _, y, _, hxy⟩ := h in ⟨⟨x, y, hxy⟩⟩

@[simp] lemma nontrivial_univ_iff : (univ : set α).nontrivial ↔ nontrivial α :=
⟨nontrivial_of_univ_nontrivial, λ h, @nontrivial_univ _ h⟩

lemma nontrivial_of_nontrivial (hs : s.nontrivial) : nontrivial α :=
let ⟨x, _, y, _, hxy⟩ := hs in ⟨⟨x, y, hxy⟩⟩

/-- `s`, coerced to a type, is a nontrivial type if and only if `s` is a nontrivial set. -/
@[simp, norm_cast] lemma nontrivial_coe_sort {s : set α} : nontrivial s ↔ s.nontrivial :=
by simp_rw [← nontrivial_univ_iff, set.nontrivial, mem_univ,
            exists_true_left, set_coe.exists, subtype.mk_eq_mk]

alias nontrivial_coe_sort ↔ _ nontrivial.coe_sort

/-- A type with a set `s` whose `coe_sort` is a nontrivial type is nontrivial.
For the corresponding result for `subtype`, see `subtype.nontrivial_iff_exists_ne`. -/
lemma nontrivial_of_nontrivial_coe (hs : nontrivial s) : nontrivial α :=
nontrivial_of_nontrivial $ nontrivial_coe_sort.1 hs

theorem nontrivial_mono {α : Type*} {s t : set α} (hst : s ⊆ t) (hs : nontrivial s) :
  nontrivial t := nontrivial.coe_sort $ (nontrivial_coe_sort.1 hs).mono hst

@[simp] lemma not_subsingleton_iff : ¬ s.subsingleton ↔ s.nontrivial :=
by simp_rw [set.subsingleton, set.nontrivial, not_forall]

@[simp] lemma not_nontrivial_iff : ¬ s.nontrivial ↔ s.subsingleton :=
iff.not_left not_subsingleton_iff.symm

alias not_nontrivial_iff ↔ _ subsingleton.not_nontrivial
alias not_subsingleton_iff ↔ _ nontrivial.not_subsingleton

protected lemma subsingleton_or_nontrivial (s : set α) : s.subsingleton ∨ s.nontrivial :=
by simp [or_iff_not_imp_right]

lemma eq_singleton_or_nontrivial (ha : a ∈ s) : s = {a} ∨ s.nontrivial :=
by { rw ←subsingleton_iff_singleton ha, exact s.subsingleton_or_nontrivial }

lemma nontrivial_iff_ne_singleton (ha : a ∈ s) : s.nontrivial ↔ s ≠ {a} :=
⟨nontrivial.ne_singleton, (eq_singleton_or_nontrivial ha).resolve_left⟩

lemma nonempty.exists_eq_singleton_or_nontrivial : s.nonempty → (∃ a, s = {a}) ∨ s.nontrivial :=
λ ⟨a, ha⟩, (eq_singleton_or_nontrivial ha).imp_left $ exists.intro a

theorem univ_eq_true_false : univ = ({true, false} : set Prop) :=
eq.symm $ eq_univ_of_forall $ classical.cases (by simp) (by simp)

section preorder
variables [preorder α] [preorder β] {f : α → β}

lemma monotone_on_iff_monotone : monotone_on f s ↔ monotone (λ a : s, f a) :=
by simp [monotone, monotone_on]

lemma antitone_on_iff_antitone : antitone_on f s ↔ antitone (λ a : s, f a) :=
by simp [antitone, antitone_on]

lemma strict_mono_on_iff_strict_mono : strict_mono_on f s ↔ strict_mono (λ a : s, f a) :=
by simp [strict_mono, strict_mono_on]

lemma strict_anti_on_iff_strict_anti : strict_anti_on f s ↔ strict_anti (λ a : s, f a) :=
by simp [strict_anti, strict_anti_on]

variables (f)

/-! ### Monotonicity on singletons -/

protected lemma subsingleton.monotone_on (h : s.subsingleton) :
  monotone_on f s :=
λ a ha b hb _, (congr_arg _ (h ha hb)).le

protected lemma subsingleton.antitone_on (h : s.subsingleton) :
  antitone_on f s :=
λ a ha b hb _, (congr_arg _ (h hb ha)).le

protected lemma subsingleton.strict_mono_on (h : s.subsingleton) :
  strict_mono_on f s :=
λ a ha b hb hlt, (hlt.ne (h ha hb)).elim

protected lemma subsingleton.strict_anti_on (h : s.subsingleton) :
  strict_anti_on f s :=
λ a ha b hb hlt, (hlt.ne (h ha hb)).elim

@[simp] lemma monotone_on_singleton : monotone_on f {a} :=
subsingleton_singleton.monotone_on f

@[simp] lemma antitone_on_singleton : antitone_on f {a} :=
subsingleton_singleton.antitone_on f

@[simp] lemma strict_mono_on_singleton : strict_mono_on f {a} :=
subsingleton_singleton.strict_mono_on f

@[simp] lemma strict_anti_on_singleton : strict_anti_on f {a} :=
subsingleton_singleton.strict_anti_on f

end preorder

section linear_order
variables [linear_order α] [linear_order β] {f : α → β}

/-- A function between linear orders which is neither monotone nor antitone makes a dent upright or
downright. -/
lemma not_monotone_on_not_antitone_on_iff_exists_le_le :
  ¬ monotone_on f s ∧ ¬ antitone_on f s ↔ ∃ a b c ∈ s, a ≤ b ∧ b ≤ c ∧
    (f a < f b ∧ f c < f b ∨ f b < f a ∧ f b < f c) :=
by simp [monotone_on_iff_monotone, antitone_on_iff_antitone, and_assoc, exists_and_distrib_left,
  not_monotone_not_antitone_iff_exists_le_le, @and.left_comm (_ ∈ s)]

/-- A function between linear orders which is neither monotone nor antitone makes a dent upright or
downright. -/
lemma not_monotone_on_not_antitone_on_iff_exists_lt_lt :
  ¬ monotone_on f s ∧ ¬ antitone_on f s ↔ ∃ a b c ∈ s, a < b ∧ b < c ∧
    (f a < f b ∧ f c < f b ∨ f b < f a ∧ f b < f c) :=
by simp [monotone_on_iff_monotone, antitone_on_iff_antitone, and_assoc, exists_and_distrib_left,
  not_monotone_not_antitone_iff_exists_lt_lt, @and.left_comm (_ ∈ s)]

end linear_order

end set

open set

namespace function

variables {ι : Sort*} {α : Type*} {β : Type*} {f : α → β}

lemma injective.nonempty_apply_iff {f : set α → set β} (hf : injective f)
  (h2 : f ∅ = ∅) {s : set α} : (f s).nonempty ↔ s.nonempty :=
by rw [nonempty_iff_ne_empty, ← h2, nonempty_iff_ne_empty, hf.ne_iff]

end function
open function

namespace set

/-! ### Lemmas about `inclusion`, the injection of subtypes induced by `⊆` -/

section inclusion
variables {α : Type*} {s t u : set α}

/-- `inclusion` is the "identity" function between two subsets `s` and `t`, where `s ⊆ t` -/
def inclusion (h : s ⊆ t) : s → t :=
λ x : s, (⟨x, h x.2⟩ : t)

@[simp] lemma inclusion_self (x : s) : inclusion subset.rfl x = x := by { cases x, refl }

lemma inclusion_eq_id (h : s ⊆ s) : inclusion h = id := funext inclusion_self

@[simp] lemma inclusion_mk {h : s ⊆ t} (a : α) (ha : a ∈ s) : inclusion h ⟨a, ha⟩ = ⟨a, h ha⟩ := rfl

lemma inclusion_right (h : s ⊆ t) (x : t) (m : (x : α) ∈ s) : inclusion h ⟨x, m⟩ = x :=
by { cases x, refl }

@[simp] lemma inclusion_inclusion (hst : s ⊆ t) (htu : t ⊆ u) (x : s) :
  inclusion htu (inclusion hst x) = inclusion (hst.trans htu) x :=
by { cases x, refl }

@[simp] lemma inclusion_comp_inclusion {α} {s t u : set α} (hst : s ⊆ t) (htu : t ⊆ u) :
  inclusion htu ∘ inclusion hst = inclusion (hst.trans htu) :=
funext (inclusion_inclusion hst htu)

@[simp] lemma coe_inclusion (h : s ⊆ t) (x : s) : (inclusion h x : α) = (x : α) := rfl

lemma inclusion_injective (h : s ⊆ t) : injective (inclusion h)
| ⟨_, _⟩ ⟨_, _⟩ := subtype.ext_iff_val.2 ∘ subtype.ext_iff_val.1

lemma eq_of_inclusion_surjective {s t : set α} {h : s ⊆ t}
  (h_surj : function.surjective (inclusion h)) : s = t :=
begin
  refine set.subset.antisymm h (λ x hx, _),
  obtain ⟨y, hy⟩ := h_surj ⟨x, hx⟩,
  exact mem_of_eq_of_mem (congr_arg coe hy).symm y.prop,
end

end inclusion

end set

namespace subsingleton

variables {α : Type*} [subsingleton α]

lemma eq_univ_of_nonempty {s : set α} : s.nonempty → s = univ :=
λ ⟨x, hx⟩, eq_univ_of_forall $ λ y, subsingleton.elim x y ▸ hx

@[elab_as_eliminator]
lemma set_cases {p : set α → Prop} (h0 : p ∅) (h1 : p univ) (s) : p s :=
s.eq_empty_or_nonempty.elim (λ h, h.symm ▸ h0) $ λ h, (eq_univ_of_nonempty h).symm ▸ h1

lemma mem_iff_nonempty {α : Type*} [subsingleton α] {s : set α} {x : α} :
  x ∈ s ↔ s.nonempty :=
⟨λ hx, ⟨x, hx⟩, λ ⟨y, hy⟩, subsingleton.elim y x ▸ hy⟩

end subsingleton

/-! ### Decidability instances for sets -/

namespace set
variables {α : Type u} (s t : set α) (a : α)

instance decidable_sdiff [decidable (a ∈ s)] [decidable (a ∈ t)] : decidable (a ∈ s \ t) :=
(by apply_instance : decidable (a ∈ s ∧ a ∉ t))

instance decidable_inter [decidable (a ∈ s)] [decidable (a ∈ t)] : decidable (a ∈ s ∩ t) :=
(by apply_instance : decidable (a ∈ s ∧ a ∈ t))

instance decidable_union [decidable (a ∈ s)] [decidable (a ∈ t)] : decidable (a ∈ s ∪ t) :=
(by apply_instance : decidable (a ∈ s ∨ a ∈ t))

instance decidable_compl [decidable (a ∈ s)] : decidable (a ∈ sᶜ) :=
(by apply_instance : decidable (a ∉ s))

instance decidable_emptyset : decidable_pred (∈ (∅ : set α)) :=
λ _, decidable.is_false (by simp)

instance decidable_univ : decidable_pred (∈ (set.univ : set α)) :=
λ _, decidable.is_true (by simp)

instance decidable_set_of (p : α → Prop) [decidable (p a)] : decidable (a ∈ {a | p a}) :=
by assumption

end set

/-! ### Monotone lemmas for sets -/

section monotone
variables {α β : Type*}

theorem monotone.inter [preorder β] {f g : β → set α}
  (hf : monotone f) (hg : monotone g) : monotone (λ x, f x ∩ g x) :=
hf.inf hg

theorem monotone_on.inter [preorder β] {f g : β → set α} {s : set β}
  (hf : monotone_on f s) (hg : monotone_on g s) : monotone_on (λ x, f x ∩ g x) s :=
hf.inf hg

theorem antitone.inter [preorder β] {f g : β → set α}
  (hf : antitone f) (hg : antitone g) : antitone (λ x, f x ∩ g x) :=
hf.inf hg

theorem antitone_on.inter [preorder β] {f g : β → set α} {s : set β}
  (hf : antitone_on f s) (hg : antitone_on g s) : antitone_on (λ x, f x ∩ g x) s :=
hf.inf hg

theorem monotone.union [preorder β] {f g : β → set α}
  (hf : monotone f) (hg : monotone g) : monotone (λ x, f x ∪ g x) :=
hf.sup hg

theorem monotone_on.union [preorder β] {f g : β → set α} {s : set β}
  (hf : monotone_on f s) (hg : monotone_on g s) : monotone_on (λ x, f x ∪ g x) s :=
hf.sup hg

theorem antitone.union [preorder β] {f g : β → set α}
  (hf : antitone f) (hg : antitone g) : antitone (λ x, f x ∪ g x) :=
hf.sup hg

theorem antitone_on.union [preorder β] {f g : β → set α} {s : set β}
  (hf : antitone_on f s) (hg : antitone_on g s) : antitone_on (λ x, f x ∪ g x) s :=
hf.sup hg

namespace set

theorem monotone_set_of [preorder α] {p : α → β → Prop}
  (hp : ∀ b, monotone (λ a, p a b)) : monotone (λ a, {b | p a b}) :=
λ a a' h b, hp b h

theorem antitone_set_of [preorder α] {p : α → β → Prop}
  (hp : ∀ b, antitone (λ a, p a b)) : antitone (λ a, {b | p a b}) :=
λ a a' h b, hp b h

/-- Quantifying over a set is antitone in the set -/
lemma antitone_bforall {P : α → Prop} : antitone (λ s : set α, ∀ x ∈ s, P x) :=
λ s t hst h x hx, h x $ hst hx

end set

end monotone

/-! ### Disjoint sets -/

variables {α β : Type*} {s t u : set α} {f : α → β}

namespace disjoint

theorem union_left (hs : disjoint s u) (ht : disjoint t u) : disjoint (s ∪ t) u :=
hs.sup_left ht

theorem union_right (ht : disjoint s t) (hu : disjoint s u) : disjoint s (t ∪ u) :=
ht.sup_right hu

lemma inter_left (u : set α) (h : disjoint s t) : disjoint (s ∩ u) t :=
h.inf_left u

lemma inter_left' (u : set α) (h : disjoint s t) : disjoint (u ∩ s) t :=
h.inf_left' _

lemma inter_right (u : set α) (h : disjoint s t) : disjoint s (t ∩ u) :=
h.inf_right _

lemma inter_right' (u : set α) (h : disjoint s t) : disjoint s (u ∩ t) :=
h.inf_right' _

lemma subset_left_of_subset_union (h : s ⊆ t ∪ u) (hac : disjoint s u) : s ⊆ t :=
hac.left_le_of_le_sup_right h

lemma subset_right_of_subset_union (h : s ⊆ t ∪ u) (hab : disjoint s t) : s ⊆ u :=
hab.left_le_of_le_sup_left h

end disjoint