be used.
        :type cmd:      ``bool``

        :param pred:    An optional predicate to pass if no command or plugin
                        predicate is set
        :type pred:     ``SoSPredicate``

        :returns:   `pred` if neither a command predicate or plugin predicate
                    is set. The command predicate if one is set and `cmd` is
                    ``True``, else the plugin default predicate (which may be
                    ``None``).
        :rtype:     ``SoSPredicate`` or ``None``
        N)