cases ha.left with
| head => rw [ha.right]; constructor
| tail x h' =>
  have h_in_tail := ih.mpr (Exists.intro a ⟨h', ha.right⟩)
  exact Mem.tail _ h_in_tail
case mpr.intro
α : Type u_1
β : Type u_2
f : α → β
b : β
x : α
xs : List α
ih : b ∈ map f xs ↔ ∃ a, a ∈ xs ∧ f a = b
a : α
ha : a ∈ x :: xs ∧ f a = b
⊢ b ∈ f x :: map f xs
no goals