exact List.Mem.head _
case mp.inr.a.inr.head
α : Type u_1
inst✝ : DecidableEq α
x y : α
as zs : List α
ih : ∀ (cs : List α), length (uniqueAux zs (cs ++ x :: (as ++ [y]))) = length (uniqueAux zs (cs ++ y :: x :: as))
cs : List α
h : y ∈ cs ++ x :: (as ++ [y])
hcons : y ∈ x :: as ++ [y]
⊢ y ∈ y :: x :: as
no goals