code: def _MergeOptional(self, a, b):
      
      if a and b:
        if a != b:
          raise MergeError("values must be identical if both specified "
                           "('%s' vs '%s')" % (transitfeed.EncodeUnicode(a),
                                               transitfeed.EncodeUnicode(b)))
      return a or b
docs: Tries to merge two values which may be None.
  
      If both values are not None, they are required to be the same and the
      merge is trivial. If one of the values is None and the other is not None,
      the merge results in the one which is not None. If both are None, the merge
      results in None.
  
      Args:
        a: The first value.
        b: The second value.
  
      Returns:
        The merged value.
  
      Raises:
        MergeError: If both values are not None and are not the same.