proposition: Lemma mcoeffMrw (d1 d2 : {fset K}) g1 g2 k : msupp g1 `<=` d1 -> msupp g2 `<=` d2 -> (g1 * g2)@_k = \sum_(k2 <- d2) \sum_(k1 <- d1) (g1@_k1 * g2@_k2) *+ (k1 * k2 == k)%M.
proof: Proof. by move=> le1 le2; rewrite (mcoeffMlw _ le1 le2) exchange_big. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(seq path choice finset fintype finfun) mathcomp(tuple bigop ssralg ssrint ssrnum) xfinmap