def find(self, header):
        """Return the index of the header in the set or return -1 if not found.

        :param header: the header to be looked up.
        """
        header = header.lower()
        for idx, item in enumerate(self._headers):
            if item.lower() == header:
                return idx
        return -1
Return the index of the header in the set or return -1 if not found.

        :param header: the header to be looked up.