Fixpoint cond_eq (T: nat -> Set) n m {struct n}: forall c, T (c + n) -> T (c + m) -> Prop :=
  match n, m return forall c, T (c + n) -> T (c + m) -> Prop with
  | 0, 0 => fun c x y => x = y
  | S n', S m' => fun c x y => cond_eq T n' m' (S c)
      (eq_rec_r T x (plus_n_Sm c n'))
      (eq_rec_r T y (plus_n_Sm c m'))
  | _, _ => fun _ _ _ => True
  end.

Lemma cond_eq_eq (T: nat -> Set) n c (x y: T (c + n)): cond_eq T n n c x y = (x = y).