ActionScript 3.0 :: Error 5000: The Class 'Particle' Must Subclass 'flash.display.MovieClip' Since It Is Linked To A Library Symbol Of That Type

Running Flash CS3, I've reviewed the lynda.com tutorial on building particle systems in ActionScript 3.0 to try and build some shooting sparks in an animation.I'm getting stuck at what I think is a really elementary point, but I can't seem to get past it. 

Basically, all I'm trying to do at this point is create a class called Particle (which I will teach to do things that particles do a little later) link it to a MovieClip on the stage in my .fla file and use the particle's update function to move it across the stage in a manner defined in the instance (so I can later give different movement properties to each AS generated instance of the MC).  I know there are easier ways to accomplish my base task without loading an external package, but this seems so strightforward that I must be doing something very simple wrong I just can't see so I thought I'd post the code here and see what I'm missing.
 
I start with a file called Particle.as in the same folder as my Spark Test.fla file.
 
In Particle.as I have the following code:
 
package{ import flash.display.*; public class Particle extends MovieClip {  public xVelocity:Number;  public yVelocity:Number;    public function Particle()  {   xVelocity = 0;   yVelocity = 0;  }    public function update():void  {   this.x += xVelocity;   this.y += yVelocity;     } }}
 
So I'm declaring that all particles are built with no motion, but if they are explicitly granted an x or y velocity, they will move by that much when the instance's update function is called in the timeline.
 
Then in my Spark Test.fla file, I have a small shape in a MovieClip called Spark.  In the Linkage properties of Spark I have chosen to Export for ActionScript and Export in FIrst Frame.  The class is called Spark and the Base class is Particle.  For testing purposes I've dropped an instance on the stage and called it spark.  Then in the actions layer in the first frame (the timeline is just the spark layer and the actions layer, one frame long each with no "stop();" so it should run forever) I have included the following code:
 
spark.xVelocity = 5;spark.yVelocity = -1;
function updateSpark(event:Event):void{ spark.update();}
addEventListener(Event.ENTER_FRAME, updateSpark);
 
What it looks like this should do is on each ENTER_FRAME Flash should call updateSpark which is a function that runs the update() event for the spark instance which is a instance of a movieclip that has an update event which moves x and y position by their relative velocity variables.  This all seems pretty straightforward to me, but when I ctrl-Enter to test the movie, I get the following 3 compiler errors:
 
Particle.as Line 1: 5000: The class 'Particle' must subclass 'flash.display.MovieClip' since it is linked to a library symbol of that type.
Particle.as Line 7: 1071: Syntax error: expected a definition keyword (such as function) after attribute public, not xVelocity.
Particle.as Line 7: 1084: Syntax error: expecting rightbrace before semicolon.