sub merge {
    my( $av, $a, $b, $c, $d )= @_;
    my $beg= $a;
    my @i;
    while(  $a <= $b  &&  $c <= $d  ) {
        my $cmp= compare( $av->[$a], $av->[$c] );
        push @i, $a++
            if  -1 != $cmp;
        push @i, $c++
            if  1 != $cmp;
    }
    @$av[$beg..$d]= @$av[ @i, $a..$b, $c..$d ];
}

sub uniqmerge {
    my( $av, $a, $b, $c, $d )= @_;
    my $beg= $a;
    my $end= $d;
    my @i;
    while(  $a <= $b  &&  $c <= $d  ) {
        my $cmp= compare( $av->[$a], $av->[$c] );
        push @i, $a++
            if  -1 != $cmp;
        push @i, $c++
            if  -1 == $cmp;
        $c++, $end--
            if  0 == $cmp;
    }
    @$av[$beg..$end]= @$av[ @i, $a..$b, $c..$d ];
    return $d-$end;
}