sub fast_merl {
        my ($source, $chop) = @_;
        local($_);

        my %found;
        $found{$_}++ while ($_ = chop($source)) ne '';
        while (($_ = chop($chop)) ne '') {
                return if --$found{$_} < 0;
        }

        return join "", map { $_ x $found{$_} } keys %found;
}