∀ {α : Type u} {cmp : α → α → Ordering} {t : Std.TreeSet.Raw α cmp} {δ : Type w} {m : Type w → Type w'} [inst : Monad m]
  [LawfulMonad m] {f : α → δ → m (ForInStep δ)} {init : δ}, forIn t init f = forIn t.toList init f