proposition: Lemma fmorph_unit x : (f x \in unit) = (x != 0).
proof: Proof. have [-> |] := eqVneq x; first by rewrite rmorph0 unitr0. by rewrite -unitfE; apply: rmorph_unit. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)