my $b = $blocks[$i];
    my $pv = $hv - 1;
    $pv = $#$b   if  $pv < 0;
    my $nv = $hv + 1;
    $nv = 0   if  $#$b < $nv;
    for my $v (  $pv, $hv, $nv  ) {
        $v = $b->[$v];
    }
    my $na= atan2( $nv->[_X] - $hv->[_X], $nv->[_Y] - $hv->[_Y] );
    my $pa= atan2( $pv->[_X] - $hv->[_X], $pv->[_Y] - $hv->[_Y] );
    if(  $na < $pa  ) {
        # Not clock-wise:

##</code><code>##

    my $pv = $hv - 1;
    $pv = $#{$blo­cks[$i]}   if  $pv < 0;

##</code><code>##

    sub _X() { 1 }
    sub _Y() { 2 }

##</code><code>##

    if ($blocks[$i][$j][_Y] > $highest) {