Ltac myinv :=
  match goal with
  | [ |- In _ nil -> _ ] => let X := fresh "X" in intro X; elim X
  | [ |- In _ (ret _ _) -> _ ] =>
        let X := fresh "X" in
        intro X; elim X; clear X;
        [let EQ := fresh "EQ" in intro EQ; unfold ret in EQ; inv EQ; myinv | myinv]
  | [ |- In _ (_ :: nil) -> _ ] =>
        let X := fresh "X" in
        intro X; elim X; clear X; [let EQ := fresh "EQ" in intro EQ; inv EQ; myinv | myinv]
  | [ |- In _ (match ?x with Some _ => _ | None => _ end) -> _ ] => destruct x eqn:?; myinv
  | [ |- In _ (match ?x with false => _ | true => _ end) -> _ ] => destruct x eqn:?; myinv
  | [ |- In _ (match ?x with left _ => _ | right _ => _ end) -> _ ] => destruct x; myinv
  | _ => idtac
  end.

Lemma step_exprlist_val_list: forall m al, is_val_list al <> None -> step_exprlist al m = nil.