proposition: Lemma dom0 : dom (0 : {freeg K / R}) = [::] :> seq K.
proof: Proof. apply/perm_small_eq/uniq_perm => //; first exact: uniq_dom. by move=> z; rewrite mem_dom coeff0 eqxx. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop order generic_quotient) mathcomp(ssralg ssrnum ssrint)