sub windex {
	my ($s1, $s2) = @_;
	my $pos = -1;
	my $null = chr 0;
	my $len = length $s1;
	while( ($pos = index $s1, $null, $pos+1) >= 0 ){
		substr $s1, $pos, 1, substr($s2, $pos, 1);
	}
	return $s1;
}