proposition: Lemma morphim_pair1g (H2 : {set gT2}) : pair1g @* H2 = setX 1 H2.
proof: Proof. by rewrite -imset2_pair imset2_set1l morphimEsub ?subsetT. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)