So, a particle will exchange its velocity (dx, dy, dz) with every other particle. That means that when you finish one cycle of exchanges, a particle will retain the velocity of the last particle it happened to interact with. The order of the interaction can make quite a difference, then. I would say, the FOR EACH doesn't give you a very fine control over the order in which the particles interact. Perhaps you should use an array to have a better control over this order of exchanges.

>Every monad has an X, Y, and Z, plus a change in X, Y, and Z.
>
>The rules are that first every monad updates its X, Y, and Z based on its dX, dY, and dZ.
>
>Then, the monad exchanges its trajectories with every other monad. Here's the relevant code:
>
>

>	procedure DoStuff
>	
>		* move inertially
>		this.nX = this.nX + this.nDx
>		this.nY = this.nY + this.nDy
>		this.nZ = this.nZ + this.nDz
>		
>		* see if there's anything to interact with
>		for each oB in oAbsoluteMatter
>			lnDx = oB.nDx
>			lnDy = oB.nDy
>			lnDz = oB.nDz
>			oB.nDx = this.nDx
>			oB.nDy = this.nDy
>			oB.nDz = this.nDz
>			this.nDx = lnDx
>			this.nDy = lnDy
>			this.nDz = lnDz
>		endfor 
>	
>	endproc
>