anchor.diff:  
   (defparameter *to-json-indent-increase* 2)
   
  -(defgeneric to-json (object &key indent)
  +(defgeneric to-json (object &key indent first-indent)
     (:documentation "Conversion of OBJECT to JSON"))
   
   (defun gen-indent (nb-chars)
  
positive.code:   (:documentation "Representation of IPython messages"))
  
  
  (defmethod to-json ((object message) &key (indent 0))
    (with-slots (header parent-header metadata content) object
      (concatenate
       'string
       (gen-indent indent) (format nil "{~%")
       (gen-indent indent) (format nil "  \"header\": ~A~%"
                                   (to-json header :indent (+ indent *to-json-indent-increase*)))
       (gen-indent indent) (format nil "  \"parent_header\": ~A~%"
                                   (to-json parent-header :indent (+ indent *to-json-indent-increase*)))
       (gen-indent indent)(format nil "  \"metadata\": ~A~%"
  
negative.code: (babel:string-to-octets
   (to-json *header1*) :encoding :utf-8)
  
  (defclass message ()
    ((header :initarg :header :accessor message-header :type header)
     (parent-header :initarg :parent-header :initform nil :accessor message-parent-header :type header)
  