Section BACKWARD_SIMULATION_PLUS.
Hypothesis simulation:
forall s2 t s2', Step L2 s2 t s2' ->
forall s1, match_states s1 s2 -> safe L1 s1 ->
exists s1', Plus L1 s1 t s1' /\ match_states s1' s2'.

Lemma backward_simulation_plus: backward_simulation L1 L2.