def add_atom_selection_arg(parser):
    """
    Add selection optional argument.

    Selection argument is a string that defines the atom selection,
    this is defined by ``-l`` and ``--selection``, and defaults to ``all``.

    Parameters
    ----------
    parser : `argparse.ArgumentParser <https://docs.python.org/3/library/argparse.html#argparse.ArgumentParser>`_
        The argument parser to which add the selection argument.
    """
    parser.add_argument('-l', '--selection', help=
        "Atom selection for the output trajectory. Selection rules are as defined by the MD analysis library used by the client interface. For instructions read the main command-line client description. Defaults to 'all'."
        , default='all', type=str)
