code: public ExampleSet copy()
  	{
  		ExampleSet copy = new SentenceSetCopy();
  		for (int i=0;i<size();i++)
  			copy.add(x(i), y(i), id(i));
  
  		return copy;
  	}
docs: Returns a shallow copy of this ExampleSet instance:
  the instances, ids and class labels themselves are
  not cloned.
  
  @return a shallow copy of this example set.