proposition: Lemma addrKA z x y : (x + z) - (z + y) = x - y.
proof: Proof. by rewrite opprD addrA addrK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)