anchor.diff:  (defgeneric (setf event-handler) (event-handler identifier)
     (:documentation "Set a new EVENT-HANDLER instance for a given IDENTIFIER.")
     (:method ((event-handler event-handler) (identifier symbol))
  -    (assert (eq identifier (identifier event-handler)) () "The identifier of the EVENT-HANDLER has to match.")
  +    (assert (eq identifier (identifier event-handler)) ()
  +            "The identifier of the EVENT-HANDLER does not match (~a != ~a)" identifier (identifier event-handler))
       (setf (gethash identifier *evt-map*) event-handler)
       (generate-handler-priority-cache)))
   
   (defun remove-event-handler (identifier)
  -  "Remove an EVENT-HANDLER associated with the given IDENTIFIER."
  +  "Remove the EVENT-HANDLER associated with the given IDENTIFIER."
     (assert (symbolp identifier) () "IDENTIFIER has to be a symbol.")
     (remhash identifier *evt-map*)
     (generate-handler-priority-cache)
  
positive.code: 
  (defclass event-handler ()
    ((%event-type :initarg :event-type :initform 'event :accessor event-type)
     (%priority :initarg :priority :initform 0 :accessor priority)
     (%identifier :initarg :identifier :initform (error "Identifier required.") :accessor identifier)
     (%handler-function :initarg :handler-function :initform (error "Handler function required.") :accessor handler-function)
     (%docstring :initarg :docstring :initform NIL :accessor docstring))
    (:documentation "Container class representing an event handler."))
  
  
negative.code: 
  (defun set-handler-function (identifier event-class function &key (priority :MAIN) docstring)
    "Set a new handler function for an event class.
  
  IDENTIFIER  --- A symbol identifying your handler.
  EVENT-CLASS --- The type of event class you want to handle.
  FUNCTION    --- The function object to dispatch to on event handling. Has to
  