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