Decorator to add sets of required mutually exclusive args to subcommands.

def argset(name, *args, **kwargs):
    """
    Decorator to add sets of required mutually exclusive args to subcommands.
    """
    def _arg(f):
        if not hasattr(f, '_subcommand_argsets'):
            f._subcommand_argsets = {}
        f._subcommand_argsets.setdefault(name, []).append((args, kwargs))
        return f
    return _arg