proposition: Lemma fgmulEr1 g1 g2 : fgmul g1 g2 = \sum_(k2 <- msupp g2) fgmul g1 << g2@_k2 *g k2 >>.
proof: Proof. by rewrite fgmulr; apply/eq_bigr=> k _; rewrite fgmulgU. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(seq path choice finset fintype finfun) mathcomp(tuple bigop ssralg ssrint ssrnum) xfinmap