sub uniqmergesort {
    my( $av, $beg, $end )= @_;
    if(  $beg < $end-1  ) {
        my $mid= int( ($beg+$end)/2 );
        my $d1= uniqmergesort( $av, $beg, $mid );
        #^^^^^^
        my $d2= uniqmergesort( $av, $mid+1, $end );
        #^^^^^^
        $d1 += uniqmerge( $av, $beg, $mid-$d1, $mid+1, $end-$d2 );
        #^^^^^ ^^^^                      ^^^^              ^^^^
        return $d1+$d2;
        #^^^^^^^^^^^^^^
    }
    if(  $beg == $end-1  ) {
        my $cmp= compare( $av->[$beg], $av->[$end] );
        if(  -1 == $cmp  ) {
            @$av[$beg,$end]= @$av[$end,$beg];
            return 0;
            #^^^^^^^^
        }
        return 1  if  0 == $cmp;
        #^^^^^^^^^^^^^^^^^^^^^^^
    }
    return 0;
    #^^^^^^^^
}