persistable sclass PersistableOKOrError<A> implements IF0<A> {
  A value;
  gettable PersistableThrowable error;
  
  *() {}
  *(A *value) {}
  *(bool dummy, PersistableThrowable *error) { assertNotNull(error); }
  
  bool ok aka isOK() { ret error == null; }
  
  toString {
    ret ok()
      ? /*"OK: " +*/ str(value)
      : "Error: " + str(error);
  }
  
  public A get() { ret !ok() ? null : value; }
  
  public A getMandatory() { if (!ok()) throw error.asRuntimeException(); ret value; }
  
  static <A> PersistableOKOrError<A> ok(A a) { ret new PersistableOKOrError(a); }
  static PersistableOKOrError error(PersistableThrowable error) { ret new PersistableOKOrError(false, error); }
}

Began life as a copy of #1012580