anchor.diff:  
   (defmethod unsubscribe ((sub object) (obj object) state)
     (if (not (subscribed? sub obj state))
  -      (error "not suscribed to state")
  +      (error 'error-invalid)
         (setf (gethash state
   		     (subscriptions-of obj))
   	    (delete sub (gethash state
  
positive.code: 
  (defmethod subscribe ((sub object) (obj object) state notification)
    (if (subscribed? sub obj state)
        (error "already subscribed to state")
        (not (null (push `(,sub . ,notification)
  		       (gethash state
  				(subscriptions-of obj)))))))
  
negative.code: 
  (defpackage :org.altervista.rjack.de-sim.core
    (:nicknames :de-sim.core)
    (:use :common-lisp :de-sim.conditions)
    (:export :imminent-event-time
  	   :subscribed? :subscribe :unsubscribe
  	   :n-event-choosed :n-destroy
  