Section Isomorphism.
Let gzZ_lone (Y : {group gTK}) :
Y \subset 'Z(K) -> gz @* 'Z(H) \isog Y -> gz @* 'Z(H) = Y.
Variables (rT : finGroupType) (GH GK G : {group rT}).
Hypotheses (defG : GH \* GK = G) (ziGHK : GH :&: GK = 'Z(GH)).
Hypothesis AutZHfull : Aut_in (Aut H) 'Z(H) \isog Aut 'Z(H).
Hypotheses (isoGH : GH \isog H) (isoGK : GK \isog K).

(* Uses gzZ_lone *)
Lemma cprod_by_uniq :
exists f : {morphism G >-> cprod_by},
[/\ isom G C f, f @* GH = CH & f @* GK = CK].