     
     /**
      *  If <code>viewport</code> is not null, 
-     *  change the vertical scroll position for line up or line down by 
+     *  changes the vertical scroll position for a line up or line 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()     

    
    
    /**
     *  Updates the value property and, if <code>viewport</code> is non null, then sets 
     *  its <code>verticalScrollPosition</code> to <code>value</code>.
     * 
     *  @param value The new value of the <code>value</code> property. 
     *  @see #viewport
     *  
     *  @langversion 3.0
     *  @playerversion Flash 10
