Remark estep_not_val:
  forall f a k e m t S, estep ge (ExprState f a k e m) t S -> is_val a = None.

(* Specifications: *)
Theorem do_step_complete: forall w S t S' w', possible_trace w t w' -> Csem.step ge S t S' -> exists rule, In (TR rule t S') (do_step w S).