proposition: Lemma ZpS_sum x y: inc x BZp -> inc y BZp -> inc (x +z y) BZp.
proof: Proof. move => pa pb; rewrite (BZsum_pp pa pb); apply BZ_of_natp_i. move: (BZ_valN (BZp_sBZ pa)) (BZ_valN (BZp_sBZ pb)) => sa sb. by apply:NS_sum. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)