proposition: Lemma fgmulEl1 g1 g2 : fgmul g1 g2 = \sum_(k1 <- msupp g1) fgmul << g1@_k1 *g k1 >> g2.
proof: Proof. by apply/eq_bigr=> k _; rewrite fgmulUg. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(seq path choice finset fintype finfun) mathcomp(tuple bigop ssralg ssrint ssrnum) xfinmap