docs: def validate_row_lengths(fields,  # type: Sequence[FieldSpec]
                           data  # type: Sequence[Sequence[str]]
                           ):
      # type: (...) -> None
      """ Validate the `data` row lengths according to the specification
          in `fields`.
  
          :param fields: The `FieldSpec` objects forming the
              specification.
          :param data: The rows to check.
          :raises FormatError: When the number of entries in a row does
              not match expectation.
      """
      for i, row in enumerate(data):
          if len(fields) != len(row):
              msg = 'Row {} has {} entries when {} are expected.'.format(
                  i, len(row), len(fields))
              raise FormatError(msg)