code: public double sqDifference(int index, double val1, double val2) {
      double val = difference(index, val1, val2);
      return val*val;
    }
docs: Returns the squared difference of two values of an attribute.
  
  @param index	the attribute index
  @param val1	the first value
  @param val2	the second value
  @return		the squared difference