Section FACTOR_BACKWARD_SIMULATION.
Variable L1: semantics.
Variable L2: semantics.
Context index order match_states (sim: bsim_properties L1 L2 index order match_states).
Hypothesis L1single: single_events L1.
Hypothesis L2wb: well_behaved_traces L2.
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).

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').