anchor.diff:            #\<  (low (| (get-label ! :required? required?) 0))
             #\>  (high (| (get-label ! :required? required?) 0))
             (get-label-undirected x :required? required?))))))
  +
  +(defun get-labels ()
  +  *next-labels*)
  
positive.code: 
  (defvar *previous-labels* nil)
  (defvar *next-labels*     nil)
  (defvar *label-changed?* nil)
  
  (defun clear-labels ()
  
negative.code:     (& 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)))))
  
  