unfold ltT, olist in |- *; simpl in |- *; auto.
intros l a b.
rewrite (fP_app l (pX a (pX b pO))); simpl in |- *; auto.
intros H'.
elim (dist_Desc_concat _ ltM (fP l) (consA (T2M a) (consA (T2M b) nilA)));
[ intros H'5 H'6; try exact H'6 | idtac ]; auto.
simple inversion H'6.
discriminate H.
discriminate H.
elim (app2_inv y x (T2M a) (T2M b) l0); [ intros H'8 H'9 | idtac ]; auto.
rewrite H'9; rewrite H'8; auto.
Qed.

Theorem olist_cons :
forall (l : list _) a b, ltT b a -> olist (pX b l) -> olist (pX a (pX b l)).