ActionScript 2.0 :: Implementing SetInterval In A Prototype?

I'm trying to write a prototype function that will use setInterval. The following script defines a simple movieclip prototype that traces the clip's x position when called. The function works fine when called explicitly ( _root.foo.trace_x(); ), but does not when called by a loop.There are three clips on the stage, their instance names are foo, man, and chu. I created an array so I could loop through and have each one run the trace_x function once.Here's the code:

Code:

clips = Array('foo', 'man', 'chu');
num_clips = clips.length;[code]....

but instead, the output is "undefined" over and over and over.An example .fla can be found at http:[url]....