code: public DSet union(DSet otherSet)
      {
          DSetImpl result = new DSetImpl(getPBKey());
          result.addAll(this);
          result.addAll(otherSet);
          return result;
      }
docs: Create a new <code>DSet</code> object that is the set union of this
  <code>DSet</code> object and the set referenced by <code>otherSet</code>.
  @param	otherSet	The other set to be used in the union operation.
  @return	A newly created <code>DSet</code> instance that contains the union of the two sets.