def METHOD_NAME(self, sys_to_be_added):
    from numpy import int_ as npint
    n_systems = len(self._systems)  # Total number of systems from mixed-in class
    if isinstance(sys_to_be_added, (int, npint)):
        # 1. If they are indices themselves, check range
        assert (
            -n_systems <= sys_to_be_added < n_systems
        ), "Rod index {} exceeds number of registered rodtems".format(
            sys_to_be_added
        )
        sys_idx = sys_to_be_added
    elif self._check_type(sys_to_be_added):
        # 2. If they are rod objects (most likely), lookup indices
        # index might have some problems : https://stackoverflow.com/a/176921
        try:
            sys_idx = self._systems.index(sys_to_be_added)
        except ValueError:
            raise ValueError(
                "Rod {} was not found, did you append it to the system?".format(
                    sys_to_be_added
                )
            )
    return sys_idx