proposition: Lemma morphim_dprodm A B : A \subset H -> B \subset K -> dprodm @* (A * B) = fH @* A * fK @* B.
proof: Proof. exact: morphim_cprodm. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)