Help with "sweeping bar effect"... ActionScript 2.0

[diegocami]

Hi everyone!, I need some directions on how to get my "sweeping bar effect" to work, I´ll try to explain myself:

Situation:
I have a couple of buttons that should attach (that´s the method I´m using)different movieclips (one each) to an empty "mc_container" on stage.
There is an attached movieclip by default (let´s call it "mc_1") .
So when the user clicks on the "button_2" for example, there´s a bar that "sweeps" over (or covers progressively, I don´t know the exact words) "mc_1" and "uncover" "mc_2".

Problem/s:
I cant´find the way to tell flash to attach the next movieclip at the exact moment in which the present movieclip is covered completely by this "sweeping bar" (which has a "big black patch" in charge of covering the movieclip on stage) before this bar starts going backwards and uncover the movieclip)...
I don´t know if I made myself clear...:-( Well, in anycase I could be more specific of course, and send the files if necessary.

ANY SUGGESTIONS ?

Thanks a lot in advance.
Diegocami.