anchor.diff:      (& required? prev next
          (error "Label ~A appears in previous and later code. Please specify a direction." x))
       (| prev next
  +       (cdr (assoc x *imported-labels*))
          (& required?
             (error "Label ~A is not defined." x)))))
   
  
positive.code:           #\<  (low (| (get-label ! :required? required?) 0))
            #\>  (high (| (get-label ! :required? required?) 0))
            (get-label-undirected x :required? required?))))))
  
negative.code: 
  (defvar *previous-labels* nil)
  (defvar *next-labels*     nil)
  (defvar *imported-labels* nil)
  (defvar *label-changed?* nil)
  
  (defun clear-labels ()
  