proposition: Lemma fgmulrw (d1 d2 : {fset K}) g1 g2 : msupp g1 `<=` d1 -> msupp g2 `<=` d2 -> fgmul g1 g2 = \sum_(k2 <- d2) \sum_(k1 <- d1) g1 *M_[k1, k2] g2.
proof: Proof. by move=> le_d1 le_d2; rewrite (fgmullw le_d1 le_d2) 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