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concept G22Variable extends ConceptWithChangeListeners is IPersistenceInfo {
  S name;
  settableWithVar volatile O value;
  settable bool persistent;
  
  // big = value is stored in separate file
  settable bool big;
  
  // field "value" is persisted iff field "persistent" is set
  public Map _persistenceInfo() { ret litmap(value := persistent); }
  
  O get() { ret value; }
  
  void setValueIfNull(O defaultValue) {
    if (defaultValue != null && value == null) value(defaultValue);
  }
}