∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.DTreeMap α β 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 fun a d => f a.fst d) = forIn t.keys init f