code: static void bottomTwoColumns(NodeInfo first, NodeInfo second, List<NodeInfo> column0, List<NodeInfo> column1) {
  		column0.add(first);
  		column0.add(second);
  
  		NodeInfo a = selectClosestN(first,second);
  		if( a == null ) {
  			return;
  		}
  		a.marked = true;
  		column1.add(a);
  		NodeInfo b = second;
  
  		while( true ) {
  			NodeInfo t = selectClosestN(a,b);
  			if( t == null ) break;
  			t.marked = true;
  			column1.add(t);
  
  			a = t;
  			t = selectClosestN(a,b);
  			if( t == null ) break;
  			t.marked = true;
  			column0.add(t);
  			b = t;
  		}
  	}
docs: Traverses along the first two columns and sets them up