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