Fixpoint constm_list {A : Type} {M : Type -> Type} `{Monad M} (xM : M A) (n : nat) : M (list A) :=
  match n with
  | O => mreturn (@nil A)
  | S n' =>
      x <- xM ;;
      xs <- constm_list xM n' ;;
      mreturn (cons x xs)
  end.

Lemma constm_list_length {A} {M} `{PredLift M} (m : M A) n : plift (fun _ => True) m -> plift (fun p => length p = n) (constm_list m n).