∀ {α β : Type w} [inst : Std.Iterator α Id β] [Std.Iterators.Finite α Id] [inst_2 : Std.IteratorLoop α Id Id]
  [Std.LawfulIteratorLoop α Id Id] [inst_4 : Std.IteratorLoop α Id Id] [Std.LawfulIteratorLoop α Id Id]
  {it : Std.Iter β}, it.length = (forIn it 0 fun x acc => pure (ForInStep.yield (acc + 1))).run