proposition: Lemma fscomp0f : left_id [fsfun] fsfun_comp.
proof: Proof. by move=> f; apply/fsfunP=> k; rewrite fscompE /= fsfun0E. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)