proposition: Lemma sdprod_compl G K H : K ><| H = G -> H \in [complements to K in G].
proof: Proof. by case/sdprodP=> _ mulKH _ tiKH; apply/complP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)