refine Nat.succ_lt_succ ?_
as : List Int
head neck : Int
tail : List Int
h : head :: neck :: tail ≠ []
⊢ List.length
      (((toDiffs (neck :: tail) (_ : ¬neck :: tail = [])).fst - head) ::
        (toDiffs (neck :: tail) (_ : ¬neck :: tail = [])).snd) <
    List.length (head :: neck :: tail)
as : List Int
head neck : Int
tail : List Int
h : head :: neck :: tail ≠ []
⊢ List.length (toDiffs (neck :: tail) (_ : ¬neck :: tail = [])).snd < List.length (neck :: tail)