public function getNewXref(): string
    {
        // Lock the row, so that only one new XREF may be generated at a time.
        DB::table('site_setting')
            ->where('setting_name', '=', 'next_xref')
            ->lockForUpdate()
            ->get();

        $prefix = 'X';

        $increment = 1.0;
        do {
            $num = (int) Site::getPreference('next_xref') + (int) $increment;

            // This exponential increment allows us to scan over large blocks of
            // existing data in a reasonable time.
            $increment *= 1.01;

            $xref = $prefix . $num;

            // Records may already exist with this sequence number.
            $already_used =
                DB::table('individuals')->where('i_id', '=', $xref)->exists() ||
                DB::table('families')->where('f_id', '=', $xref)->exists() ||
                DB::table('sources')->where('s_id', '=', $xref)->exists() ||
                DB::table('media')->where('m_id', '=', $xref)->exists() ||
                DB::table('other')->where('o_id', '=', $xref)->exists() ||
                DB::table('change')->where('xref', '=', $xref)->exists();
        } while ($already_used);

        Site::setPreference('next_xref', (string) $num);

        return $xref;
    }