exact Mem.tail _ h_in_tail
case mpr.intro.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
a : α
ha : a ∈ x :: xs ∧ f a = b
h' : Mem a xs
h_in_tail : b ∈ map f xs
⊢ b ∈ f x :: map f xs
no goals