induction L <;> simp [filter] at *
α : Type u_1
inst✝ : SizeOf α
f : α → Bool
L : List α
h : filter f L ≠ L
⊢ sizeOf (filter f L) < sizeOf L
case cons
α : Type u_1
inst✝ : SizeOf α
f : α → Bool
head✝ : α
tail✝ : List α
tail_ih✝ : ¬filter f tail✝ = tail✝ → sizeOf (filter f tail✝) < sizeOf tail✝
h :
  ¬(match f head✝ with
      | true => head✝ :: filter f tail✝
      | false => filter f tail✝) =
      head✝ :: tail✝
⊢ sizeOf
      (match f head✝ with
      | true => head✝ :: filter f tail✝
      | false => filter f tail✝) <
    1 + sizeOf head✝ + sizeOf tail✝