[PD] GEM: How to do this?

[type_non]

This weekend i saw a great concert/video interaction by Portuguese artists Miguel Carvalhais, Pedro Tudela (@c )and Lia. During the concert there was a screen where some 2D forms reacted to music. And one of the most intriguing effects i saw was this:
 
- imagine little white squares falling in the screen, like rain. But, at a certain moment, the squares would leave a trace, or in other words, the square falls a path from top to bottom but as the square falls, the previous square position would be present. It's like a "delay" or echo effect forming a vertical line from top to bottom. Then, as some other squares with a different color apears (grey), it deletes the previous paintings done by the other squares. 
 
It's hard to me to explain, so try to look at this link: http://www.at-c.org/?p=30x1.3.
 
I know how to move squares in GEM and follow a path and that. But as i move the square, the previous position disappeas. I want to "freeze" the time, so to speak...
 
Now, is there any object in GEM that allows this? If not, could you give me some hints or ideias of how to do this? i really like the ideia and i want to explore this and adapt to my own ideas.
 
Thanks in advance.

		
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