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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def getYadisXRD(xrd_tree):
 """Return the XRD element that should contain the Yadis services"""

    xrd = None

    # for the side-effect of assigning the last one in the list to the
    # xrd variable
    for xrd in xrd_tree.findall(xrd_tag):
        pass

    # There were no elements found, or else xrd would be set to the
    # last one
    if xrd is None:
        raise XRDSError('No XRD present in tree')

    return xrd