proposition: Lemma fsetDl (T : choiceType) (A C : {fset T}) k : k \in A `\` C -> k \in A.
proof: Proof. by case/fsetDP. Qed.
imports: mathcomp(all_ssreflect) Coq(Relations.Relation_Definitions) GraphTheory(edone preliminaries) GraphTheory(bij)