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