anchor.diff:  
   (defgeneric write-command (command stream)
     (:documentation "Write the gnuplot COMMAND to STREAM. Commands
  -represented by the classes PLOT, SET-COMMAND, etc."))
  +  represented by the classes PLOT, SET-COMMAND, etc."))
   
   (defgeneric write-mapping (mapping stream)
     (:documentation "Write the gnuplot MAPPING to STREAM."))
  
positive.code:                          (output '*standard-output*) (error '*error-output*))
                          &body body)
    "Start gnuplot, bind STREAM and *COMMAND-STREAM* to its standard
  input. The stream is closed when BODY exits."
    `(with-command-stream ((start-session :display ,display :geometry ,geometry
                                          :persistp ,persistp :output ,output
                                          :error ,error))
  
negative.code: 
  (defvar *command-stream* nil
    "The default stream to which commands and inline data are written by
    WRITE-COMMAND.")
  
  (defmacro with-command-stream ((stream) &body body)
    "Binds *COMMAND-STREAM* to STREAM routing all command output to
    STREAM by default."
    `(let ((*command-stream* ,stream))
       ,@body))
  
  