intros x
case h
α : Type u_1
f : Filter α
⊢ ∀ (x : Set α), x ∈ (Basis.as_filter (as_basis f)).sets ↔ x ∈ f.sets
case h
α : Type u_1
f : Filter α
x : Set α
⊢ x ∈ (Basis.as_filter (as_basis f)).sets ↔ x ∈ f.sets