proposition: Fact Im_is_additive : additive Im.
proof: Proof. by move=> x y; rewrite !ImE rmorphB opprD addrACA -opprD mulrBr mulrBl. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)