(** Forward simulation for expressions. *)

Lemma estep_simulation:
forall S1 t S2, Cstrategy.estep ge S1 t S2 ->
forall S1' (MS: match_states S1 S1'),
exists S2',
(plus step1 tge S1' t S2' \/
(star step1 tge S1' t S2' /\ measure S2 < measure S1)%nat)
/\ match_states S2 S2'.