proposition: Lemma in_union_extract_4 : forall x l1 l2 l3 l4, x \in (l1 \u l2 \u l3 \u \{x} \u l4).
proof: Proof. Proof using. intros. apply in_union_get_4. apply in_single_self. Qed.
imports: TLC(LibTactics LibLogic LibReflect LibRelation LibList LibInt LibNat LibOperation LibEpsilon LibSet LibMonoid)