proposition: Lemma joing_idl A B : <<A>> <*> B = A <*> B.
proof: Proof. by rewrite -!(joingC B) joing_idr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)