I just learned about prettify-symbols mode and
prettify-symbols-unprettify-at-point from Tony Zorman's recent blog post,
so I tried it out, unsuccessfully at first...

Recipe from emacs -Q:

In the *scratch* buffer:
M-x prettify-symbols-mode
(setq prettify-symbols-unprettify-at-point t)
C-x C-e ;; to evaluate the setq
lambda  ;; the symbol λ appears
C-b

At this point, I'm expecting the symbol λ to deprettify back to
"lambda", but it does not.

Looking in the code in prog-mode.el, I see that the unprettification
needs to be set **before** the minor mode is activated in the buffer.
Perhaps this should be pointed out in the documentation?