@classmethod
def core(cls, num):
    """Get the :py:class:`.Routes` for the numbered core.

        Raises
        ------
        ValueError
            If the core number isn't in the range 0-17 inclusive.
        """
    if not 0 <= num <= 17:
        raise ValueError('Cores are numbered from 0 to 17')
    return cls(6 + num)
