def add_plane_selection_arg(parser):
    """
    Add plane selection argument.

    Plane selection is a selection of three regions separated by 'or'
    operator.
    
    Is defined by ``-z`` and ``--plane-selection``.

    Parameters
    ----------
    parser : `argparse.ArgumentParser <https://docs.python.org/3/library/argparse.html#argparse.ArgumentParser>`_
        The argument parser to which add the plane-selection argument.
    """
    parser.add_argument('-z', '--plane-selection', help=
        "Three selection strings representing three atom regions. The plane is defined by the three centres of geometry of the three selections. For example: -z 'segid A' 'segid B'  'segid C'."
        , required=True, nargs=3)
