apply List.mem_append_back
case mpr.inr.a.head.a
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 ++ y :: x :: as
⊢ Mem y (as ++ [y])
case mpr.inr.a.head.a.a
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 ++ y :: x :: as
⊢ y ∈ [y]