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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 at_xpath(self, xpath, timeout = DEFAULT_AT_TIMEOUT, **kw):
 """ Returns the first node matching the given XPath 2.0 expression or ``None`` if a timeout occurs. """

    return self.wait_for_safe(lambda: super(WaitMixin, self).at_xpath(xpath),
                              timeout = timeout,
                              **kw)