proposition: Lemma S_pulb_law4 : Pullback_law4 S_pulb_p S_pulb_q S_pulb_diese.
proof: Proof. unfold Pullback_law4, Pullback_eq2 in |- *; simpl in |- *. unfold Ext in |- *; simpl in |- *; intros D t1 t2 H h H1 H2 x. unfold Equal_Sprod in |- *; simpl in |- *; split. apply Sym; apply H1. apply Sym; apply H2. Qed.
imports: Pullbacks Setoid_prop SET SetoidPROD