(** Exploiting determinacy *)
Remark silent_or_not_silent:
forall t, t = E0 \/ t <> E0.
Remark not_silent_length:
forall t1 t2, (length (t1 ** t2) <= 1)%nat -> t1 = E0 \/ t2 = E0.

Lemma f2b_determinacy_star:
forall s s1, Star L2 s E0 s1 ->
forall t s2 s3,
Step L2 s1 t s2 -> t <> E0 ->
Star L2 s t s3 ->
Star L2 s1 t s3.