Remark copy_trans_parent :
 forall s3 s4 : Site, parent (copy_trans c0 s1 s2) s4 s3 -> parent c0 s4 s3.

Lemma copy_trans_parent_cr : forall s3 s4 : Site, (parent c0 s3 s4 -> dt c0 s3 < dt c0 s4) -> parent (copy_trans c0 s1 s2) s3 s4 -> dt (copy_trans c0 s1 s2) s3 < dt (copy_trans c0 s1 s2) s4.