∀ {α : Type u} {β : Type v} {cmp : α → α → Ordering} {t : Std.ExtTreeMap α β 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 δ} {init : δ},
  Std.ExtTreeMap.foldrM (fun a x d => f a d) init t = List.foldrM f init t.keys