sub combs2 {
    my( $z, $o ) = @_;
    my $str = '0'x$z . '1'x$o;
    my $l = length $str;
    return sub {
        my $p = 1+rindex( $str, '01' ) or return;
        my $r = 1+rindex( $str, '1');
        my $s = 1+rindex( $str, '0', $r-1);
        substr( $str, $s, 0 ) = substr( $str, $r, $l-$r, '' );
        my $q = index( $str, '1', $p );
        substr( $str, $p-1, 1 ) = '1';
        substr( $str, $q,   1 ) = '0';
        $str;
    };
}