anchor.diff:    "Return data last access across all its nodes, regardless of the owner.
   Return Epoch if DATA is not found or if entry has no timestamp."
     (let ((nodes (find-nodes history data)))
  -    (if nodes
  -        (the (values local-time:timestamp &optional)
  +    (the (values local-time:timestamp &optional)
  +         (if nodes
                (apply #'local-time:timestamp-maximum
  -                    (mapcar #'last-access nodes)))
  -        (local-time:unix-to-timestamp 0))))
  +                    (mapcar #'last-access nodes))
  +             (local-time:unix-to-timestamp 0)))))
   
   (define-class owner ()
     ;; TODO: Add slot pointing to history an owner belongs to?  Unnecessary if we never expose the `owner' to the caller.
  
positive.code: (defun make-entry (history data)
    "Return an `entry' wrapping DATA and suitable for HISTORY."
    (make-instance 'entry :value data
                          :key (key history)
                          :test (test history)
                          :hash-function (hash-function history)))
  
  (define-class node ()
    ((parent nil
  
negative.code: 
  ;; TODO: Store data-last-access in entry (max of all last-accesses) so that we
  ;; keep this information even for node-less entries.
  (define-class entry ()
    ((history nil
              :type (or null history-tree)
              :documentation "Required.
  This is gives access to the custom hash functions, see the corresponding
  `history-tree' slots.
  We allow null values for easier deserialization.")
     (value nil
            :type t
            :documentation "Arbitrary data."))
  