code: public InstanceList[] nextSplit() {
          InstanceList[] ret = new InstanceList[2];
          ret[0] = new InstanceList(pipe);
          for (int i = 0; i < folds.length; i++) {
              if (i == index)
                  continue;
              Iterator<Instance> iter = folds[i].iterator();
              while (iter.hasNext())
                  ret[0].add(iter.next());
          }
          ret[1] = folds[index].shallowClone();
          index++;
          return ret;
      }
docs: Returns the next training/testing split.
  
  @return A pair of lists, where <code>InstanceList[0]</code> is the larger
  split (training) and <code>InstanceList[1]</code> is the smaller
  split (testing)