How to give slide effect in guitar

by UtilZone 
Using SiON we can produce digital sounds of various instruments. But I'm trying to find a way to give the effect of sliding a guitar string from high to low or low to high frequency. In other word; when decay starts I want the frequency of the sound to be increased or decreased continuously.

Using the setPitchModulation function; it produce similar effect but the frequency increases and decreases alternatively. I want the this either decrease or increase.
♥0 | Line 33 | Modified 2011-03-26 23:09:44 | MIT License
play

ActionScript3 source code

/**
 * Copyright UtilZone ( http://wonderfl.net/user/UtilZone )
 * MIT License ( http://www.opensource.org/licenses/mit-license.php )
 * Downloaded from: http://wonderfl.net/c/jU7u
 */

package {
    import flash.display.Sprite;
    import flash.events.*;
    import com.bit101.components.*;
    import org.si.sion.*;
    import org.si.sion.sequencer.SiMMLTrack;
    import org.si.sion.utils.SiONPresetVoice;

    public class FlashTest extends Sprite {
        private var track:SiMMLTrack;
        private var driver:SiONDriver;
        private var presetVoice:SiONPresetVoice;
        private var voice:SiONVoice;
        public function FlashTest() {
           driver = new SiONDriver();
           presetVoice = new SiONPresetVoice();
           voice = new SiONVoice();
           voice = presetVoice["valsound.guitar"][1];
           driver.play();

           var btn:PushButton = new PushButton(this);
            btn.label = "Play!";
            btn.x = 10;
            btn.y = 10
            btn.addEventListener(MouseEvent.CLICK, function(e:MouseEvent):void {
                track = driver.noteOn(60, voice, 0, 0, 0, 0, true);
                track.noteShift  = 5;
                track.pitchShift = 100;
                track.velocity = 500;
                track.expression = 10;
                track.channel.pan = 10;
                //track.channel.setPitchModulation(10);
            }); 
        }
    }
}

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