anchor.diff:  (defclass channel ()
     ())
   
  +(defgeneric description (source))
   (defgeneric source-type (source))
   (defgeneric open-source (type identifier &key &allow-other-keys))
   (defgeneric close-source (source))
  
positive.code: (defmethod print-object ((channel channel) stream)
    (print-unreadable-object (channel stream :type T)
      (format stream "~s ~s" :identifier (identifier channel))))
  
negative.code: (defgeneric payload (event))
  
  ;; Registration
  (define-storage source 'equal)
  
  (defmethod open-source :around (type identifier &key)
    (let ((source (call-next-method)))
  