intros A B0 B x H' H'0 H'1; try assumption.
apply in_eq_part.
intros x0 H'2; try assumption.
apply in_diff.
apply in_part_comp_r with (add_part B0 x).
cut (in_part x0 B0).
unfold add_part in |- *.
auto with *.
apply diff_in_l with B; auto with *.
auto with *.
apply diff_in_r with B0; auto with *.
intros x0 H'2; try assumption.
elim (classic (Equal x0 x)).
intros H'3; try assumption.
absurd (in_part x B); auto with *.
cut (in_part x (diff A B)).
intros H'4; try assumption.
apply diff_in_r with A; auto with *.
apply in_part_trans_eq with x0; auto with *.
intros H'3; try assumption.
apply in_diff.
apply add_part_in_el_diff with x; auto with *.
apply in_part_comp_r with A; auto with *.
apply diff_in_l with B; auto with *.
apply diff_in_r with A; auto with *.
Qed.

Lemma diff_not_in :
forall (A B : part_set E) (x : E), ~ in_part x A -> ~ in_part x (diff A B).