>> (defun texinfo-prettify-symbols-compose-p (start end _match)
>>    ;; We know the matches all start with a backslash and end with
>>    ;; a word-element.
>>    (not (or (memq (char-before start) '(?\\))
>>             (memq (char-syntax (or (char-after end) ?\s)) '(?w))
>>             (nth 8 (syntax-ppss)))))
>> (add-hook 'texinfo-mode-hook
>>            (lambda ()
>>              (push '("\\alpha" . ?α) prettify-symbols-alist)
>>              (push '("\\beta" . ?β) prettify-symbols-alist)
>>              (setq prettify-symbols-compose-predicate 'texinfo-prettify-symbols-compose-p)
>>              (prettify-symbols-mode)))
>
> So I have to add the following defun in prog-mode.el?
>
> (defun texinfo-prettify-symbols-compose-p (start end _match)
>

I don't know what you're trying to do.  If you're trying to do something 
just for yourself, just add the above lines in your .emacs file.  If you 
want to improve texinfo-mode, the defun above should probably go into 
texinfo.el, with the "(setq prettify-symbols-compose-predicate 
'texinfo-prettify-symbols-compose-p)" inside the "(define-derived-mode 
texinfo-mode ...".