def add_trajectories_arg(parser):
    """
    Add trajectory positional argument to parser.
    
    Accepts multiple trajectory files.

    Parameters
    ----------
    parser : `argparse.ArgumentParser <https://docs.python.org/3/library/argparse.html#argparse.ArgumentParser>`_
        The argument parser to add the trajectory positionl argument.
    """
    parser.add_argument('trajectories', help=
        'Path to the trajectory files. If multiple files are given, trajectories will be concatenated by input order.'
        , nargs='+')
