         
     /**
      *  If <code>viewport</code> is not null, 
-     *  change the vertical scroll position for page up or page down by 
+     *  changes the vertical scroll position for a page up or page down operation 
+     *  by 
      *  scrolling the viewport.
      *  This method calculates the amount to scroll by calling the 
      *  <code>IViewport.getVerticalScrollPositionDelta()</code> method 

     *  to the appropriate value.
     *
     *  <p>If <code>viewport</code> is null, 
     *  change the vertical scroll position for page up or page down by calling 
     *  the <code>changeValueByPage()</code> method.</p>
     *
     *  @param increase Whether the page scroll is up (<code>true</code>) or
     *  down (<code>false</code>). 
     * 
     *  @see mx.components.baseClasses.TrackBase#changeValueByPage()
     *  @see mx.components.baseClasses.TrackBase#setValue()
     *  @see spark.core.IViewport
     *  @see spark.core.IViewport#verticalScrollPosition
     *  @see spark.core.IViewport#getVerticalScrollPositionDelta()     

 *  <p>Although you can use the VScrollBar control as a stand-alone control,
 *  you usually combine it as part of another group of components to
 *  provide scrolling functionality.</p>
 *  @mxml
 *  <p>The <code>&lt;VScrollBar&gt;</code> tag inherits all of the tag 
 *  attributes of its superclass and adds the following tag attributes:</p>
 *
 *  <pre>
 *  &lt;VScrollBar
 *
 *    <strong>Properties</strong>
 *    viewport=""
 *  /&gt;
 *  </pre>
 *  
 *  @see spark.skins.spark.VScrollBarSkin
 *  @see spark.skins.spark.VScrollBarThumbSkin
 *  @see spark.skins.spark.VScrollBarTrackSkin
 *  @langversion 3.0
 *  @playerversion Flash 10
 *  @playerversion AIR 1.5
