Corollary forward_simulation_same_safe_behavior:
  forall L1 L2, forward_simulation L1 L2 ->
  forall beh,
  program_behaves L1 beh -> not_wrong beh ->
  program_behaves L2 beh.Definition safe_along_behavior (s: state L1) (b: program_behavior) : Prop :=
  forall t1 s' b2, Star L1 s t1 s' -> b = behavior_app t1 b2 ->
     (exists r, final_state L1 s' r)
  \/ (exists t2, exists s'', Step L1 s' t2 s'').Remark safe_along_safe:
  forall s b, safe_along_behavior s b -> safe L1 s.Remark star_safe_along:
  forall s b t1 s' b2,
  safe_along_behavior s b ->
  Star L1 s t1 s' -> b = behavior_app t1 b2 ->
  safe_along_behavior s' b2.Remark not_safe_along_behavior:
  forall s b,
  ~ safe_along_behavior s b ->
  exists t, exists s',
     behavior_prefix t b
  /\ Star L1 s t s'
  /\ Nostep L1 s'
  /\ (forall r, ~(final_state L1 s' r)).

(* Specifications: *)
Lemma backward_simulation_star: forall s2 t s2', Star L2 s2 t s2' -> forall i s1 b, match_states i s1 s2 -> safe_along_behavior s1 (behavior_app t b) -> exists i', exists s1', Star L1 s1 t s1' /\ match_states i' s1' s2'.
Lemma backward_simulation_forever_silent: forall i s1 s2, Forever_silent L2 s2 -> match_states i s1 s2 -> safe L1 s1 -> Forever_silent L1 s1.
Lemma backward_simulation_forever_reactive: forall i s1 s2 T, Forever_reactive L2 s2 T -> match_states i s1 s2 -> safe_along_behavior s1 (Reacts T) -> Forever_reactive L1 s1 T.
Lemma backward_simulation_state_behaves: forall i s1 s2 beh2, match_states i s1 s2 -> state_behaves L2 s2 beh2 -> exists beh1, state_behaves L1 s1 beh1 /\ behavior_improves beh1 beh2.
Theorem backward_simulation_behavior_improves: forall L1 L2, backward_simulation L1 L2 -> forall beh2, program_behaves L2 beh2 -> exists beh1, program_behaves L1 beh1 /\ behavior_improves beh1 beh2.