class Node{ var [mixin=Node]left,right; var v;
   fcn init(val,[Node]l=Void,[Node]r=Void) { v,left,right=vm.arglist }
}

class BTree{ var [mixin=Node] root;
   fcn init(r){ root=r }
   const VISIT=Void, LEFT="left", RIGHT="right";
   fcn preOrder  { traverse(VISIT,LEFT, RIGHT) }
   fcn inOrder   { traverse(LEFT, VISIT,RIGHT) }
   fcn postOrder { traverse(LEFT, RIGHT,VISIT) }
   fcn [private] traverse(order){  //--> list of Nodes
      sink:=List();
      fcn(sink,[Node]n,order){
         if(n){ foreach o in (order){
	    if(VISIT==o) sink.write(n);
	    else self.fcn(sink,n.setVar(o),order);  // actually get var, eg n.left
	 }}
      }(sink,root,vm.arglist);
      sink
   }
   fcn levelOrder{  // breadth first
      sink:=List(); q:=List(root);
      while(q){
         n:=q.pop(0); l:=n.left; r:=n.right;
	 sink.write(n);
	 if(l) q.append(l);
	 if(r) q.append(r);
      }
      sink
   }
}