'raise *o* to the top of the stack and return *o*.  *o* must be
in the stack'

def bubble(self, o):

    if (o not in self._elements):
        raise ValueError(u'Unknown    element    o')
    old = self._elements[:]
    self.clear()
    bubbles = []
    for thiso in old:
        if (thiso == o):
            bubbles.append(thiso)
        else:
            self.push(thiso)
    for thiso in bubbles:
        self.push(o)
    return o
