def add_topology_arg(parser):
    """
    Add topology positional argument to parser.

    Parameters
    ----------
    parser : `argparse.ArgumentParser <https://docs.python.org/3/library/argparse.html#argparse.ArgumentParser>`_
        The argument parser to add the topology positionl argument.
    """
    parser.add_argument('topology', help='Path to the topology file.', type=str
        )
