proposition: Lemma rmorph_unit x : x \in unit -> f x \in unit.
proof: Proof. case/unitrP=> y [yx1 xy1]; apply/unitrP. by exists (f y); rewrite -!rmorphM // yx1 xy1 rmorph1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)