proposition: Lemma rmorph_div x y : y \in unit -> f (x / y) = f x / f y.
proof: Proof. by move=> Uy; rewrite rmorphM /= rmorphV. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)