(** Yet another alternative definition of [forever]. *)
CoInductive forever_plus (ge: genv) : state -> traceinf -> Prop :=
| forever_plus_intro: forall s1 t s2 T1 T2,
plus ge s1 t s2 ->
forever_plus ge s2 T2 ->
T1 = t *** T2 ->
forever_plus ge s1 T1.

Lemma forever_plus_forever:
forall ge s T, forever_plus ge s T -> forever ge s T.