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