rewrite (le_plus_minus z y);trivial.
rewrite minus_plus;rewrite plus_comm;rewrite <- minus_plus_simpl_l_reverse;trivial.
Qed.

(* Specifications: *)
Lemma minus_minus_lemma2 : forall (x y z:nat),(x-y-z)=(x-(y+z)).
induction x;simpl;trivial.