What's the difference between these inner codes in VOPs?

[magneto]

Hi,

While looking at some of the built-in VOPs, I noticed some of their inner code has code like this:

#if !strcmp($signature, "v4")
   float $cr = vop_bias($val.x, $amount.x);
   float $cg = vop_bias($val.y, $amount.y);
   float $cb = vop_bias($val.z, $amount.z);
   float $ca = vop_bias($val.w, $amount.w);
   $bias = set($cr, $cg, $cb, $ca);[/CODE]

while others have:

[CODE]
float $tt, $dbias;
$dbias = vop_FilterWidth($bias);
$tt = FILTERSTEP($thresh12, $bias - $dbias - $blur12, $bias + $dbias + $blur12, $filter);
$tt += FILTERSTEP($thresh23, $bias - $dbias - $blur23, $bias + $dbias + $blur23, $filter);
if ($tt > 1) $blend = lerp($region2, $region3, $tt-1);
else $blend = lerp($region1, $region2, $tt);[/CODE]

Do #if, #else instead of if, else allow conditional compilation?

Is it possible to have a VEX function that won't be called if a particular argument has a certain value?

Thanks.