Compose a string showing the optional variables (helper for the
        API printing function)

def optional_namestring(self):
        """
        Compose a string showing the optional variables (helper for the
        API printing function)
        """
        opt_names = list(self.optional_names())
        if opt_names:
            return '[, {}]'.format(', '.join(opt_names))
        else:
            return ''