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: (defclass command ()
    ()
    (:documentation "A gnuplot command. There are separate classes for
  plot/splot, etc."))
  
  (defclass plot ()
    ((kind :type (member :2d :3d) :initform :2d :initarg :kind :reader kind)
  
negative.code:   ((options :type (or list string) :initform nil :initarg :options
              :reader options))
    (:documentation "A plot can contain multiple mappings. For example
    in
  
      plot sin(x) title 'sine', \"datafile\" with lines
  
    there is a function and a file mapping. Subclasses of MAPPING
    represent the function or the data and the options (e.g. 'title',
    'with' above)."))
  
  (defclass function-mapping (mapping)
    ((expression :initarg :expression :reader function-expression)))
  