feat(data/finset): add lemma for empty filter (#4188)

A little lemma, analogous to `filter_true_of_mem` to make it convenient to reduce a filter which always fails.

src/data/finset/basic.lean

@@ -932,11 +932,14 @@ by ext; simp
 @[simp] theorem filter_false {h} (s : finset α) : @filter α (λa, false) h s = ∅ :=
 ext $ assume a, by simp only [mem_filter, and_false]; refl
 
-/-- If all elements of a `finset` satisfy the predicate `p`, `s.filter
-p` is `s`. -/
+/-- If all elements of a `finset` satisfy the predicate `p`, `s.filter p` is `s`. -/
 @[simp] lemma filter_true_of_mem {s : finset α} (h : ∀ x ∈ s, p x) : s.filter p = s :=
 ext $ λ x, ⟨λ h, (mem_filter.1 h).1, λ hx, mem_filter.2 ⟨hx, h x hx⟩⟩
 
+/-- If all elements of a `finset` fail to satisfy the predicate `p`, `s.filter p` is `∅`. -/
+lemma filter_false_of_mem {s : finset α} (h : ∀ x ∈ s, ¬ p x) : s.filter p = ∅ :=
+eq_empty_of_forall_not_mem (by simpa)
+
 lemma filter_congr {s : finset α} (H : ∀ x ∈ s, p x ↔ q x) : filter p s = filter q s :=
 eq_of_veq $ filter_congr H
 

src/data/monoid_algebra.lean

@@ -90,7 +90,7 @@ calc (f * g) x = (∑ a₁ in f.support, ∑ a₂ in g.support, F (a₁, a₂))
 ... = ∑ p in (f.support.product g.support).filter (λ p : G × G, p.1 * p.2 = x), f p.1 * g p.2 :
   (finset.sum_filter _ _).symm
 ... = ∑ p in s.filter (λ p : G × G, p.1 ∈ f.support ∧ p.2 ∈ g.support), f p.1 * g p.2 :
-  sum_congr (by { ext, simp [hs, and_comm] }) (λ _ _, rfl)
+  sum_congr (by { ext, simp only [mem_filter, mem_product, hs, and_comm] }) (λ _ _, rfl)
 ... = ∑ p in s, f p.1 * g p.2 : sum_subset (filter_subset _) $ λ p hps hp,
   begin
     simp only [mem_filter, mem_support_iff, not_and, not_not] at hp ⊢,