proposition: Lemma BZsum_le2l a b c: intp a -> intp b -> intp c -> ((c +z a) <=z (c +z b) <-> a <=z b).
proof: Proof. move => pa pb pc. apply: (iff_trans (zle_diffP (ZSs pc pa) (ZSs pc pb))). apply: iff_sym; apply: (iff_trans (zle_diffP pa pb)); by rewrite BZdiff_sum_simpl_l. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)