∀ {α : Type u} {cmp : α → α → Ordering} {t : Std.ExtTreeSet α cmp} {δ : Type w} {m : Type w → Type w'}
  [inst : Std.TransCmp cmp] [inst_1 : Monad m] [inst_2 : LawfulMonad m] {f : α → δ → m (ForInStep δ)} {init : δ},
  forIn t init f = forIn t.toList init f