DUSTBM.c = DUST Box Model
						   pi r^2 = area O
						   len*width = 4rs^2 = area O

			   width=2*rs
		   /-------------------------------/ h=h2
		 /                          *    /  |
     len=2*rs  /      .   +    :               /    |  nl=4
	     /                       '       /      |
	   /     *        .     o          / vel[3]/
	 /           `             ,     /   /   /  |
       /                               /   /   /    |  nl=3
     /-------------------------------/   /   /      |
    |      /                    o    | /   /        |    slab=h2/nl
    |    /       *        .        , |   /         /
    |  /             `               | /  vel[2] /  |
    |/-------------------------------/     /   /    |  nl=2
    /-------------------------------/    /   /      |
    |      /                    o    | /   /        |     pdz=1/nl
    |    /       *        .       ,  |   /         /
    |  /             `               | /  vel[1] /  |
    |/-------------------------------/    /    /    |  nl=1
    /-------------------------------/   /    /      |
    |      /                    o    |     /        |
    |    /       *        .       ,  |   /         / h=0
    |  /             `               | /         /
    |/-------------------------------/   vel[0]/
    /-------------------------------/   /    /
    |      /                    o    |     /
    |    /       *         .      ,  |   /
    |  /             `               | /
    |/-------------------------------/
	    Dust Cloud Source



	  dx=(gxf-gxo+rs)/ng                     pdx=dx/len
    -------------+-------------------------------------------------------
   |                             |                                       |
   |                             |                                       |
 w |        .<- dx  ->.          .         .         .         .         |
 i |                             |                                       |
 d |<---- rs --->o<----- rs ---->|                                       |
 t |        .         .          .         .         .         .         |
 h |        ^           Dust     |                                       | pdy=
   |       dy          Cloud     |                                       | dy/width
   |        .         .          .         .         .         .         |
   |<---------- len ---------->  |                                       |
   |                             |                                       |
    -------------+-------------------------------------------------------
 -rs           gxo=0            rs       ^ use for xg ^                 gxf
		       xg=xn-rs     xn=xg+rs
    <------------------------- gxf-gxo+rs ------------------------------>
  xn=0                         use for xn                            xn=gxf+rs



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	       o             O
	      o o           O O       Fluff Factor
	       o             O