(** Completeness part 1: [step_expr] contains all possible non-error reducts. *)

Lemma rred_topred:
forall w' r1 m1 t r2 m2,
rred ge r1 m1 t r2 m2 -> possible_trace w t w' ->
exists rule, step_expr RV r1 m1 = topred (Rred rule r2 m2 t).