match h : f a a' with 
| true => simp [insert, h]
| false => simp [insert, h]; apply len_insert_eq_succ_len
α : Type
f : α → α → Bool
r : α → α → Prop
a : α
l : List α
a' : α
as : List α
⊢ List.length (insert f a (a' :: as)) = List.length (a' :: as) + 1
no goals