∀ {m : Type w → Type u_1} {α β : Type w} [inst : Std.Iterator α m β] [Std.Iterators.Finite α m] [inst_2 : Monad m]
  [LawfulMonad m] [inst_4 : Std.IteratorLoop α m m] [Std.LawfulIteratorLoop α m m] {it : Std.IterM m β},
  it.length = (fun x => { down := x.length }) <$> it.toList