∀ {α β : Type w} [inst : Std.Iterator α Id β] {m : Type w → Type w'} [inst_1 : Monad m]
  (f : β → Std.Iterators.PostconditionT m (ULift.{w, 0} Bool)) (it : Std.Iter β),
  Std.Iter.filterWithPostcondition f it = Std.IterM.filterWithPostcondition f it.toIterM