<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _MergeEntities(self, a, b):
 """Merges two stops. For the stops to be merged, they must have: - the same stop_id - the same stop_name (case insensitive) - the same zone_id - locations less than largest_stop_distance apart The other attributes can have arbitary changes. The merged attributes are taken from the new stop. Args: a: The first stop. b: The second stop. Returns: The merged stop. Raises: MergeError: The stops could not be merged. """

    distance = transitfeed.ApproximateDistanceBetweenStops(a, b)
    if distance > self.largest_stop_distance:
      raise MergeError("Stops are too far apart: %.1fm "
                       "(largest_stop_distance is %.1fm)." %
                       (distance, self.largest_stop_distance))
    scheme = {'stop_id': self._MergeIdentical,
              'stop_name': self._MergeIdenticalCaseInsensitive,
              'zone_id': self._MergeIdentical,
              'location_type': self._MergeIdentical}
    return self._SchemedMerge(scheme, a, b)