apply Nat.le_trans
case cons.inr
α : Type u_1
inst✝ : DecidableEq α
x : α
xs : List α
ih : ∀ (as : List α), length (uniqueAux xs as) ≤ length xs + length as
as : List α
h : ¬x ∈ as
⊢ length (uniqueAux xs (x :: as)) ≤ length xs + 1 + length as
case cons.inr.a
α : Type u_1
inst✝ : DecidableEq α
x : α
xs : List α
ih : ∀ (as : List α), length (uniqueAux xs as) ≤ length xs + length as
as : List α
h : ¬x ∈ as
⊢ length (uniqueAux xs (x :: as)) ≤ ?cons.inr.m

case cons.inr.a
α : Type u_1
inst✝ : DecidableEq α
x : α
xs : List α
ih : ∀ (as : List α), length (uniqueAux xs as) ≤ length xs + length as
as : List α
h : ¬x ∈ as
⊢ ?cons.inr.m ≤ length xs + 1 + length as

case cons.inr.m
α : Type u_1
inst✝ : DecidableEq α
x : α
xs : List α
ih : ∀ (as : List α), length (uniqueAux xs as) ≤ length xs + length as
as : List α
h : ¬x ∈ as
⊢ Nat