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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 update(self):
 """Update the position of the mark in the collection. :return: this object, for chaining :rtype: Mark """

        rec = self._c.find_one({}, {self._fld: 1}, sort=[(self._fld, -1)], limit=1)
        if rec is None:
            self._pos = self._empty_pos()
        elif not self._fld in rec:
            _log.error("Tracking field not found. field={} collection={}"
                       .format(self._fld, self._c.name))
            _log.warn("Continuing without tracking")
            self._pos = self._empty_pos()
        else:
            self._pos = {self._fld: rec[self._fld]}
        return self