code: private UnconditionalValueDerefSet duplicateFact(UnconditionalValueDerefSet fact) {
          UnconditionalValueDerefSet copyOfFact = createFact();
          copy(fact, copyOfFact);
          fact = copyOfFact;
          return fact;
      }
docs: Return a duplicate of given dataflow fact.
  
  @param fact
  a dataflow fact
  @return a duplicate of the input dataflow fact