anchor.diff:  ;; I/O functions
   
   (defun body-to-list (body)
  +  "Converts a body into a list of its attributes"
     (list (x (pos body))
   	(y (pos body))
   	(x (vel body))
  
positive.code: 	(id body)))
  
  (defun bodies-to-list (bodies)
    (mapcar #'body-to-list bodies))
  
  (defun save-list (lst filename)
    (with-open-file (out filename
  		       :direction :output
  		       :if-exists :supersede)
  
negative.code:       (print lst out))))
  
  (defun save-bodies (filename)
    "Calls save-list and body-to-list"
    (save-list (bodies-to-list *bodies*) filename))
  
  (defun read-list (filename)
    "Reads from a file with the name given"
    (with-open-file (in filename)
      (with-standard-io-syntax
        (read in))))
  
  (defun read-bodies (filename)
    "Reads from a file and converts the lists in the file
     into instances of the body class"
    (loop for bod in (read-list filename)
         collecting (make-instance 
  		   'body
  