code: def _cursor_pb(cursor_pair):
      
      if cursor_pair is not None:
          data, before = cursor_pair
          value_pbs = [_helpers.encode_value(value) for value in data]
          return query_pb2.Cursor(values=value_pbs, before=before)
docs: Convert a cursor pair to a protobuf.
  
      If ``cursor_pair`` is :data:`None`, just returns :data:`None`.
  
      Args:
          cursor_pair (Optional[Tuple[list, bool]]): Two-tuple of
  
              * a list of field values.
              * a ``before`` flag
  
      Returns:
          Optional[google.cloud.firestore_v1beta1.types.Cursor]: A
          protobuf cursor corresponding to the values.