Code:

...
var mouse_wheel = 0; 
     mouse_wheel = sys_metrics(75); // see if the mouse has a scrolling wheel or not 
     while (1) 
     { 
          if (mouse_wheel != 0) // wheel present? 
          { 
               // then use the wheel to zoom in and out 
               camera.arc -= 0.1 * mickey.z * time; 
          } 
          else // wheel not detected 
          { 
               // use the "Y" and "H" keys to zoom in and out 
               camera.arc -= (key_y - key_h) * time; 
          } 
          camera.arc = clamp(camera.arc,20,90); // limit camera.arc to 20..90 degrees 
          wait (1); 
     } 
...