proposition: Lemma cfker_isom phi : cfker (cfIsom phi) = f @* cfker phi.
proof: Proof. rewrite unlock cfker_morph // defG cfRes_id morphpre_restrm morphpre_invm. by rewrite -defR !morphimIim. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div choice fintype tuple finfun bigop prime order) mathcomp(ssralg poly finset fingroup morphism perm) mathcomp(automorphism quotient finalg action gproduct) mathcomp(zmodp commutator cyclic center pgroup sylow) mathcomp(matrix vector falgebra ssrnum algC algnum) mathcomp(archimedean)