code: function computeForwardSlotSegs(levels) {
  	var i, level;
  	var j, seg;
  	var k;
  
  	for (i=0; i<levels.length; i++) {
  		level = levels[i];
  
  		for (j=0; j<level.length; j++) {
  			seg = level[j];
  
  			seg.forwardSegs = [];
  			for (k=i+1; k<levels.length; k++) {
  				computeSlotSegCollisions(seg, levels[k], seg.forwardSegs);
  			}
  		}
  	}
  }
docs: For every segment, figure out the other segments that are in subsequent levels that also occupy the same vertical space. Accumulate in seg.forwardSegs