concept G22Variable extends ConceptWithChangeListeners is IPersistenceInfo {
  S name;
  settableWithVar volatile O value;
  settable bool persistent;
  
  public Map _persistenceInfo() { ret litmap("value", persistent); }
  
  O get() { ret value; }
  
  void setValueIfNull(O defaultValue) {
    if (value == null) value(defaultValue);
  }
}