cases ih_val with | intro a ha =>
exact Exists.intro a (And.intro (Mem.tail _ ha.left) ha.right)
case mp.tail
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
h' : Mem b (map f xs)
ih_val : ∃ a, a ∈ xs ∧ f a = b
⊢ ∃ a, a ∈ x :: xs ∧ f a = b
no goals