def pole(sys):
    """
    Compute system poles.

    Parameters
    ----------
    sys: StateSpace or TransferFunction
        Linear system

    Returns
    -------
    poles: ndarray
        Array that contains the system's poles.

    Raises
    ------
    NotImplementedError
        when called on a TransferFunction object

    See Also
    --------
    zero
    TransferFunction.pole
    StateSpace.pole

    """
    return sys.pole()
