proposition: Lemma fgpos_dom D : {subset dom (fgpos D) <= dom D}.
proof: Proof. by move=> x /fgmap_dom; rewrite mem_filter => /andP []. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop order generic_quotient) mathcomp(ssralg ssrnum ssrint)