In addition to the code snippet from Arthur, I can suggest the
following:

#+begin_src elisp
(defvar lockywolf/common-mathematical-prettify-alist
  (list '("<=" . ?≤)
	'(">=" . ?≥))
  "Lockywolf's custom prettification list.")

(add-hook 'scheme-mode-hook
	  (lambda ()
	    (setq prettify-symbols-alist
	       (seq-concatenate 'list
				(list '("define" . ?≝)
				      '("set!"  . ?≐)
				      '("set-car!"  . ?≔)
				      '("set-cdr!"  . ?≕)
				      '("#t"  . ?✓)
				      '("#f"  . ?✗)
				      '("'()"  . ?∅)
				      '("if"  . ?⁇)
				      '("or"  . ?∨)
				      '("and"  . ?∧))
				lockywolf/common-mathematical-prettify-alist
				prettify-symbols-alist))
	    ))

(add-hook 'emacs-lisp-mode-hook
	  (lambda ()
	    (setq prettify-symbols-alist
	          (seq-concatenate 'list
				   (list '("defun"  . ?≝)
				         '("set"  . ?≔)
				         '("setf"  . ?≕)
				         '("setq"  . ?≐)
				         '("t"  . ?✓)
				         '("nil"  . ?⦰)
				         '("'()"  . ?∅)
				         '("if"  . ?⁇)
				         '("or"  . ?∨)
				         '("and"  . ?∧))
				   lockywolf/common-mathematical-prettify-alist
				   prettify-symbols-alist))
))

(use-package prettify-greek
  :ensure t
  :demand t)

(global-prettify-symbols-mode 1)
(setq prettify-symbols-unprettify-at-point t)
(add-hook 'prog-mode-hook
	  (lambda ()
            (setq prettify-symbols-alist
	          (seq-concatenate 'list
				   prettify-symbols-alist
				   prettify-greek-upper
				   prettify-greek-lower))))

#+end_src

You can check if it is readable for you by, for example:

#+begin_src
;;;; check prettification
(lambda (a b)
  (or (+ a b)
      (if nil t '())))
#+end_src

This list is unfinished, I would generally like to have a way to:
1. Quickly distinguish primitive forms from derived.
2. Quickly distinguish procedures from syntax.

But (1) would require reading a lot through the character table, and I
do not have a lot of time for that at the moment; (2) is do not know how
to do.

"Peter" <xxxxxx@gmx.net> writes:

>> Saying that, however, I do use Emacs fontification to substitute fancy
>> symbols in place of lambda, define, set!, set-car, set-cdr, and some
>> other built-ins.
>
> Would you mind sharing that mapping? That sounds interesting!

--
Your sincerely,
Vladimir Nikishkin (MiEr, lockywolf)
(Laptop)