anchor.diff:  
   See also: `pstream-elt-future', `phistory'"
     (assert (integerp n) (n))
  -  (unless (typep pstream 'pstream)
  +  (unless (pstream-p pstream)
       (return-from pstream-elt (pstream-elt (as-pstream pstream) n)))
     (with-slots (history history-number) pstream
       (let ((real-index (if (minusp n)
                             (+ history-number n)
                             n)))
  -      (if (and (>= real-index 0)
  -               (< real-index (length history))
  +      (if (and (>= real-index (max 0 (- history-number (length history))))
                  (< real-index history-number))
  -          (elt history real-index)
  +          (elt history (pstream-elt-index-to-history-index pstream real-index))
             (error 'pstream-out-of-range :index n)))))
   
   (defun pstream-history-advance-by (pstream index) ;; FIX: add tests for this
  
positive.code:           (if (>= index history-number)
                (- index (1- history-number))
                0)))))
  
  (defun pstream-elt-index-to-history-index (pstream index)
    "Given INDEX, an absolute index into PSTREAM's history, return the actual index into the current recorded history of the pstream.
  
  See also: `pstream-history-advance-by'"
    (assert (>= index 0) (index))
    (with-slots (history) pstream
      (mod index (length history))))
  
  (defun pstream-elt-future (pstream n)
    "Get the element N away from the most recent in PSTREAM's history. Unlike `pstream-elt', this function will automatically peek into the future for any positive N.
  
negative.code:   (:report (lambda (condition stream)
               (format stream "The index ~d falls outside the scope of the pstream's history." (pstream-elt-index condition)))))
  
  (defun pstream-elt-index-to-history-index (pstream index)
    "Given INDEX, an absolute index into PSTREAM's history, return the actual index into the current recorded history of the pstream.
  
  See also: `pstream-history-advance-by'"
    (assert (>= index 0) (index))
    (with-slots (history) pstream
      (mod index (length history))))
  
  (defun pstream-elt (pstream n)
    "Get the Nth item in PSTREAM's history. For negative N, get the -Nth most recent item.
  
  