(** One or several transitions.  Also known as the transitive closure. *)
Inductive plus (ge: genv): state -> trace -> state -> Prop :=
| plus_left: forall s1 t1 s2 t2 s3 t,
step ge s1 t1 s2 -> star ge s2 t2 s3 -> t = t1 ** t2 ->
plus ge s1 t s3.

Lemma star_inv:
forall ge s1 t s2,
star ge s1 t s2 ->
(s2 = s1 /\ t = E0) \/ plus ge s1 t s2.