proposition: Lemma isog_cprod_by : G \isog C.
proof: Proof. by have [f [isoG _ _]] := cprod_by_uniq; apply: isom_isog isoG. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(fintype bigop finset fingroup morphism perm) mathcomp(automorphism quotient action gproduct gfunctor) mathcomp(cyclic)