code: public boolean nextSplit() {
          if( numSplits == 0 )
              return false;
          x2 = splits[--numSplits];
          if( numSplits > 0 )
              x1 = splits[numSplits-1]+1;
          else
              x1 = 0;
  
          return true;
      }
docs: Tells it to process the submatrix at the next split.  Should be called after the
  current submatrix has been processed.