Return the index of the header in the set or raise an
        :exc:`IndexError`.

        :param header: the header to be looked up.

def index(self, header):
        """Return the index of the header in the set or raise an
        :exc:`IndexError`.

        :param header: the header to be looked up.
        """
        rv = self.find(header)
        if rv < 0:
            raise IndexError(header)
        return rv