Remark starN_star:
  forall n s t s', starN n s t s' -> star s t s'.Remark star_starN:
  forall s t s', star s t s' -> exists n, starN n s t s'.

(* Specifications: *)
Lemma forever_N_forever: forall a s T, forever_N a s T -> forever s T.
Lemma forever_plus_forever: forall s T, forever_plus s T -> forever s T.