diff --git a/doc/examples/tempus-perfectum-change-pitches.lsp b/doc/examples/tempus-perfectum-change-pitches.lsp
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+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; File:             tempus-perfectum-change-pitches.lsp
+;;;
+;;; Class Hierarchy:  None
+;;;
+;;; Version:          1.0
+;;;
+;;; Project:          slippery chicken (algorithmic composition)
+;;;
+;;; Purpose:          To show how pitches can be changed algorithmically using
+;;;                   post-generation techniques. 
+;;;
+;;; Author:           Michael Edwards
+;;;
+;;; Creation date:    February 19th 2024
+;;;
+;;; $$ Last modified:  11:15:15 Mon Feb 19 2024 CET
+;;;
+;;; ****
+;;; Licence:          Copyright (c) 2010 Michael Edwards
+;;;
+;;;                   This file is part of slippery-chicken
+;;;
+;;;                   slippery-chicken is free software; you can redistribute it
+;;;                   and/or modify it under the terms of the GNU General
+;;;                   Public License as published by the Free Software
+;;;                   Foundation; either version 3 of the License, or (at your
+;;;                   option) any later version.
+;;;
+;;;                   slippery-chicken is distributed in the hope that it will
+;;;                   be useful, but WITHOUT ANY WARRANTY; without even the
+;;;                   implied warranty of MERCHANTABILITY or FITNESS FOR A
+;;;                   PARTICULAR PURPOSE.  See the GNU General Public License
+;;;                   for more details.
+;;;
+;;;                   You should have received a copy of the GNU General Public
+;;;                   License along with slippery-chicken; if not, write to the
+;;;                   Free Software Foundation, Inc., 59 Temple Place, Suite
+;;;                   330, Boston, MA 02111-1307 USA
+;;; 
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+(in-package :sc)
+
+;;; or comment out and load tempus-perfectum.lsp by hand if working in another
+;;; directory
+(load-from-same-dir "tempus-perfectum.lsp")
+(proclaim '(special +tempus-perfectum+))
+
+(defmethod tendency-unison ((sc slippery-chicken)
+                            &optional
+                            (main-player 'ob)
+                            ;; x values can range over whatever you like but y
+                            ;; values are expressed as a percentage where 0 = no
+                            ;; pitch change and 100 = definite pitch
+                            ;; change. change 'em all by default, just to prove
+                            ;; it works.
+                            (change-chance-env '(0 100 100 100)))
+  (let* ((all-events (get-events-sorted-by-time sc))
+         (chance 0.0)
+         (do-it nil)
+         (main-player-note nil)
+         (event-count 0)
+         (changed-count 0))
+    ;; we've used an x-axis from 0-100 (or whatever), but need to stretch this
+    ;; over all the notes in the whole piece (including, for simplicity's sake
+    ;; the main-player
+    (setq change-chance-env (new-lastx change-chance-env (num-notes sc)))
+    (loop for event in all-events do
+             (when (needs-new-note event) ; only process attacked notes
+               (if (eq main-player (player event))
+                   ;; for simplicity's sake we'll just grab the last main-player
+                   ;; note and use that to change subsequent non-main-player
+                   ;; notes, accepting or even enjoying that, depending on
+                   ;; rhythmic structure, we might be a little behind.
+                   (setq main-player-note (pitch-or-chord event))
+                   ;; don't change main-player's notes, and only change other
+                   ;; player's notes if we've seen a main-player note already
+                   (when main-player-note
+                     ;; get the chance of changing 
+                     (setq chance (interpolate event-count change-chance-env)
+                           ;; determine whether to change the note to the
+                           ;; main-player's note using fixed-seed
+                           ;; randomness. Note that we reset the random
+                           ;; generator when event-count is 0, i.e. on the first
+                           ;; time through the loop. Note also the less-than
+                           ;; test assumes that the Y values of the
+                           ;; change-chance-env range from 0 to 100
+                           do-it (< (random-rep 100.0 (zerop event-count))
+                                    chance))
+                     (when do-it
+                       ;; but bear in mind that no checks are made here that the
+                       ;; instrument we're dealing with can actually play the
+                       ;; main player's note! If we wanted to be more
+                       ;; sophisticated, then we could use the in-range method
+                       ;; and octavise where necessary.
+                       (setf (pitch-or-chord event) main-player-note)
+                       (incf changed-count))))
+               (incf event-count)))
+    ;; bear in mind that we've only changed attacked notes' pitches, so any
+    ;; tied-to notes will be wrong! Helpfully the check-ties method takes care
+    ;; of fixing these :-)
+    (check-ties sc t)
+    ;; these two for statistics' sake really
+    (update-slots sc)
+    (update-instrument-slots sc)
+    (format t "~&Changed ~a notes." changed-count)
+    sc))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; now actually do it, cloning the original so as not to have to reload, and/or
+;;; to enable easier comparisons
+
+(let ((sc (tendency-unison (clone +tempus-perfectum+))))
+  (cmn-display sc :size 10
+                  :in-c t))
+
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;; EOF