feat(order/filter): add two lemmas (#11519)

Two easy lemmas (from the sphere eversion project) and some minor style changes.

src/order/filter/at_top_bot.lean

@@ -101,6 +101,10 @@ lemma eventually_gt_at_top [preorder α] [no_max_order α] (a : α) :
   ∀ᶠ x in at_top, a < x :=
 Ioi_mem_at_top a
 
+lemma eventually_ne_at_top [preorder α] [no_max_order α] (a : α) :
+  ∀ᶠ x in at_top, x ≠ a :=
+(eventually_gt_at_top a).mono (λ x hx, hx.ne.symm)
+
 lemma eventually_lt_at_bot [preorder α] [no_min_order α] (a : α) :
   ∀ᶠ x in at_bot, x < a :=
 Iio_mem_at_bot a

src/order/filter/basic.lean

@@ -122,16 +122,19 @@ filter.ext_iff.2
 @[simp] lemma univ_mem : univ ∈ f :=
 f.univ_sets
 
-lemma mem_of_superset : ∀ {x y : set α}, x ∈ f → x ⊆ y → y ∈ f :=
-f.sets_of_superset
+lemma mem_of_superset {x y : set α} (hx : x ∈ f) (hxy : x ⊆ y) : y ∈ f :=
+f.sets_of_superset hx hxy
 
-lemma inter_mem : ∀ {s t}, s ∈ f → t ∈ f → s ∩ t ∈ f :=
-f.inter_sets
+lemma inter_mem {s t : set α} (hs : s ∈ f) (ht : t ∈ f) : s ∩ t ∈ f :=
+f.inter_sets hs ht
 
-@[simp] lemma inter_mem_iff {s t} : s ∩ t ∈ f ↔ s ∈ f ∧ t ∈ f :=
+@[simp] lemma inter_mem_iff {s t : set α} : s ∩ t ∈ f ↔ s ∈ f ∧ t ∈ f :=
 ⟨λ h, ⟨mem_of_superset h (inter_subset_left s t),
   mem_of_superset h (inter_subset_right s t)⟩, and_imp.2 inter_mem⟩
 
+lemma diff_mem {s t : set α} (hs : s ∈ f) (ht : tᶜ ∈ f) : s \ t ∈ f :=
+inter_mem hs ht
+
 lemma univ_mem' (h : ∀ a, a ∈ s) : s ∈ f :=
 mem_of_superset univ_mem (λ x _, h x)