anchor.diff:    (multiple-value-bind (subscribers state-present?)
         (gethash state (subscriptions-of obj))
       (if (not state-present?)
  -	(error "not a valid state")
  +	(error 'error-invalid)
   	(not (null (find sub subscribers :key #'car))))))
   
   
  
positive.code: 
  (defpackage :org.altervista.rjack.de-sim.core
    (:nicknames :de-sim.core)
    (:use :common-lisp)
    (:export :imminent-event-time
  	   :subscribed? :subscribe :unsubscribe
  	   :n-event-choosed :n-destroy
  
negative.code: 
  (defmethod subscribe ((sub object) (obj object) state notification)
    (if (subscribed? sub obj state)
        (error 'error-already)
        (not (null (push `(,sub . ,notification)
  		       (gethash state
  				(subscriptions-of obj)))))))
  