Inductive fbs_match: index -> state L1 -> (trace * state L2) -> Prop :=
  | fbs_match_intro: forall i s1 t s2 s1',
      Star L1 s1 t s1' -> match_states i s1' s2 ->
      t = E0 \/ output_trace t ->
      fbs_match i s1 (t, s2).Inductive fbs_match: index -> state L1 -> (trace * state L2) -> Prop :=
  | fbs_match_intro: forall i s1 t s2 s1',
      Star L1 s1 t s1' -> match_states i s1' s2 ->
      t = E0 \/ output_trace t ->
      fbs_match i s1 (t, s2).

(* Specifications: *)
Lemma fbs_simulation: forall s2 t s2', Step (atomic L2) s2 t s2' -> forall i s1, fbs_match i s1 s2 -> safe L1 s1 -> exists i', exists s1', (Plus L1 s1 t s1' \/ (Star L1 s1 t s1' /\ order i' i)) /\ fbs_match i' s1' s2'.
Lemma fbs_progress: forall i s1 s2, fbs_match i s1 s2 -> safe L1 s1 -> (exists r, final_state (atomic L2) s2 r) \/ (exists t, exists s2', Step (atomic L2) s2 t s2').
Theorem factor_backward_simulation: forall L1 L2, backward_simulation L1 L2 -> single_events L1 -> well_behaved_traces L2 -> backward_simulation L1 (atomic L2).