proposition: Lemma BZsum_pm1 x y: inc x BZp -> inc y BZms -> (BZ_val y) <=c (BZ_val x) -> x +z y = BZ_of_nat((BZ_val x) -c (BZ_val y)).
proof: Proof. by move => pa pb pc; rewrite BZsum_pm //; Ytac0. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)