simp_rw [not_iff_not]
case h
α✝ : Type ?u.1842
α : Type u_1
f : Filter α
p q : α → Prop
h : ∀ᶠ (a : α) in f, p a ↔ q a
⊢ ∀ᶠ (x : α) in f, ¬p x ↔ ¬q x
case h
α✝ : Type ?u.1842
α : Type u_1
f : Filter α
p q : α → Prop
h : ∀ᶠ (a : α) in f, p a ↔ q a
⊢ ∀ᶠ (x : α) in f, p x ↔ q x