. apply Nat.le_trans ?_ (Nat.le_add_left _ _)
  assumption
case cons.h_2
α : Type u_1
inst✝ : SizeOf α
f : α → Bool
head✝ : α
tail✝ : List α
tail_ih✝ : sizeOf (filter f tail✝) ≤ sizeOf tail✝
x✝ : Bool
heq✝ : f head✝ = false
⊢ sizeOf (filter f tail✝) ≤ 1 + sizeOf head✝ + sizeOf tail✝
no goals