proposition: Lemma divrr : {in [pred x | val x \is a unit], right_inverse 1 invU *%R}.
proof: Proof. by move=> x /[!inE] xu; apply: val_inj; rewrite rmorphM rmorph1 /= SubK mulrV. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)