anchor.diff:  
   (defclass node ()
     ((%name :initform (gensym) :reader name)
  +   (%inputs :initform '() :accessor inputs)
  +   (%outputs :initform '() :accessor outputs)
      (%successors :initform '() :accessor successors)))
   
   (defun make-node (&optional successors)
  
positive.code: 
  ;;; The probability of a successor being in a lower layer.
  (defparameter *back-edge-probability* 0.2)
  
  (defun random-flow-chart
      (&optional
  
negative.code: 		       do (process-node node zsp))
  		 (setf layer next-layer)
  		 (setf next-layer '()))))
      (assign-random-inputs/outputs initial)
      (values initial #'inputs #'outputs)))
  			
  (defun draw-flow-chart (initial-node filename)
    (with-open-file (stream filename
  