proposition: Lemma coeff_outdom D x : x \notin dom D -> coeff x D = 0.
proof: Proof. by rewrite mem_dom negbK => /eqP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop order generic_quotient) mathcomp(ssralg ssrnum ssrint)