anchor.diff:          | Some c -> f c
           | None -> ()
   
  -        
         | For(_, a, b, c, d) ->
           f a
           f b
  
positive.code:       | Pass
        | Null
        | This
        | Undefined -> ()
         
        | Throw t
        | Return t
        | Eval t 
  
negative.code:     | New of Tree * Tree list
      | Return of Tree
      | Function of string option * Scope * Tree
      | FunctionFast of string option * Scope ref * Tree
      | Invoke of Tree * Tree list
      | Label of string * Tree
      | For of string option * Tree * Tree * Tree * Tree
  