Proof (proj1 bigstep_to_steps).

Lemma eval_expr_to_steps:
forall e m K a t m' a',
eval_expr e m K a t m' a' ->
forall C f k, leftcontext K RV C ->
simple a' = true /\ typeof a' = typeof a /\
star step ge (ExprState f (C a) k e m) t (ExprState f (C a') k e m').