Section Union1.
Variable E : Setoid.
Definition union : part_set E -> part_set E -> part_set E.
intros A B.
apply (Build_Predicate (Pred_fun:=fun x : E => in_part x A \/ in_part x B)).
red in |- *.
intros x y H' H'0; try assumption.
elim H'; [ intros H'1; try exact H'1; clear H' | intros H'1; clear H' ].
left; try assumption.
apply in_part_comp_l with x; auto with algebra.
right; try assumption.
apply in_part_comp_l with x; auto with algebra.
Defined.
unfold included in |- *; simpl in |- *; intuition.
Qed.
unfold included in |- *; simpl in |- *; intuition.
Qed.

Lemma in_part_union_l :
forall (A B : part_set E) (x : E), in_part x A -> in_part x (union A B).