Code Fragment: SubtractMerger

template <typename Object, typename Sequence>		// set subtraction
class SubtractMerger : public Merger<Object, Sequence> {
protected:
  virtual void aIsLess(const Object& a, Sequence& C)
    { C.insertLast(a); }				// add a
  virtual void bothEqual(const Object& a, const Object& b, Sequence& C)
    { } 						// ignore
  virtual void bIsLess(const Object& b, Sequence& C)
    { }							// ignore
public:
  SubtractMerger() { }
};