def _sift_down( self, ndx ):
    """ Sift the value at the ndx element down the tree. """
    left = 2 * ndx + 1
    right = 2 * ndx + 2    
    # Determine which node contains the larger value, left or right.
    largest = ndx
    if left < self._count and self._elements[left] >= self._elements[largest]:
      largest = left
    if right < self._count and self._elements[right] >= self._elements[largest]:
      largest = right
    if largest != ndx:   # one of the children holds larger value
      self._elements[largest], self._elements[ndx] = \
          self._elements[ndx], self._elements[largest]
      self._sift_down( largest )