proposition: Lemma BZdiff_sum_simpl_l: (x +z y) -z (x +z z) = y -z z.
proof: Proof. rewrite /BZdiff (BZoppD xz zz) (BZsumC x y). rewrite (BZsumA (ZSs yz xz) (ZSo xz) (ZSo zz)). by rewrite -(BZsumA yz xz (ZSo xz)) BZsum_opp_r // BZsum_0r. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)