def add_top_output_arg(parser):
    """
    Add argument to export first frame as topology PDB file.

    Defined by ``-o`` and ``--top-output``.

    Parameters
    ----------
    parser : `argparse.ArgumentParser <https://docs.python.org/3/library/argparse.html#argparse.ArgumentParser>`_
        The argument parser to which add the topology output argument.
    """
    parser.add_argument('-o', '--top-output', help=
        "Export edited trajectory first frame as topololy file. You can specify the exact file name, otherwise, defaults to input trajectory path + '_frame0.pdb'. Also, if name starts with '_', it is used as file suffix, if name ends with '_', it is used as prefix, instead."
        , default=False, const='_frame0.pdb', nargs='?')
