proposition: Lemma addmA : associative fgadd.
proof: Proof. elim/quotW=> [D1]; elim/quotW=> [D2]; elim/quotW=> [D3]. unlock fgadd; rewrite ?piE; apply/eqmodP/freegequivP => k /=. by rewrite !(precoeff_reduce, precoeff_cat, precoeff_repr) addrA. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop order generic_quotient) mathcomp(ssralg ssrnum ssrint)