apply Zplus_reg_l with a; auto.
Qed.

(* Specifications: *)
Theorem Zplus_ge_compat_l: forall n m p : Z, (n >= m -> p + n >= p + m)%Z.
intros n m p H; apply Z.le_ge; apply Zplus_le_compat_l; auto; apply Z.ge_le; auto.