sclass DerivedMechLists extends VirtualMechLists {
  new SimpleMechLists a;
  VirtualMechLists b = new DefaultMechLists;

  S mL_raw(S listName) {
    S text = a.mL_raw(listName);
    if (text != null) ret text;
    ret b.mL_raw(listName);
  }
  
  void append(S listName, S text) {
    a.append(listName, text);
  }
  
  void setText(S listName, S text) {
    a.setText(listName, text);
  }
}

Began life as a copy of #1017767