proposition: Lemma BZsum_ACA a b c d: intp a -> intp b -> intp c -> intp d -> (a +z b) +z (c +z d) = (a +z c) +z (b +z d).
proof: Proof. move => az bz cz dz; move: (ZSs cz dz) (ZSs bz dz)=> cdz bdz. by rewrite -!BZsumA // (BZsum_CA bz cz dz). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)