proposition: Lemma opprD : {morph -%R: x y / x + y : V}.
proof: Proof. by move=> x y; rewrite -[y in LHS]opprK opprB addrC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)