proposition: Lemma opprB x y : - (x - y) = y - x.
proof: Proof. by apply: (canRL (addrK x)); rewrite addrC subKr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)