elim (dist_Desc_concat _ ltM (consA (T2M a) (consA (T2M b) nilA)) (fP l));
[ intros H'5 H'6 | idtac ]; auto.
apply olist_ltT with (l := pO); auto.
Qed.

Theorem olist_X : forall (l : list _) a, olist (pX a l) -> olist l.
intros l a H'.