Section fparts_in_def.
Variable E : Setoid.
(* Add_part. *)
Definition add_part (A : part_set E) (x : E) := union A (single x).
unfold add_part in |- *; auto with algebra.
Qed.
Hint Resolve add_part_comp: algebra.

Lemma add_part_in : forall (A : part_set E) (x : E), in_part x (add_part A x).