Why is this `cursor` causing error ?

[demotomohiro]

I think following document and example code explain why your code cause error 
and would help you to find a way to fix it:

  * <https://nim-lang.org/docs/destructors.html#the-dotcursor-annotation>



Compile this code with `nim c -r --mm:orc test.nim`:
    
    
    type
      NodeObj = object
        left: Node
        right {.cursor.}: Node
        name: string
      
      Node = ref NodeObj
    
    proc `=destroy`(x: var NodeObj) =
      echo "destroy ", x.name
    
    proc main =
      var
        n0 = Node(name: "n0")
        r = Node(left: n0, right: n0, name: "r")
      
      n0 = nil
      
      echo "r.left = nil"
      r.left = nil
      
      echo "r.right = nil"
      r.right = nil
      
      echo "end of main"
    
    main()
    
    Run