proposition: Lemma in_vsetDE (G : pre_graph) x E: x \in vset G -> x \in vset (G - E).
proof: Proof. apply. Qed.
imports: HB(structures) Coq(Relation_Definitions Morphisms RelationClasses) mathcomp(all_ssreflect) GraphTheory(edone finite_quotient preliminaries bij equiv) GraphTheory(setoid_bigop structures pttdom rewriting) GraphTheory(finmap_plus)