iterator yields is a tuple of a line and a text version that that line
        is present in (not introduced in).

        Ordering of results is in whatever order is most suitable for the
        underlying storage format.

        If a progress bar is supplied, it may be used to indicate progress.
        The caller is responsible for cleaning up progress bars (because this
        is an iterator).

        NOTES:
         * Lines are normalised by the underlying store: they will all have \n
           terminators.
         * Lines are returned in arbitrary order.
         * If a requested key did not change any lines (or didn't have any
           lines), it may not be mentioned at all in the result.

        :param pb: Progress bar supplied by caller.
        :return: An iterator over (line, key).
        NFr