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;; This function is quoted from this page. Thanks to Tomohiro Matsuyama.
;; http://dev.ariel-networks.com/Members/matsuyama/pretty-lambda-in-emacs/
(defun set-pretty-patterns (patterns)
  (loop for (glyph . pairs) in patterns do
    (loop for (regexp . major-modes) in pairs do
      (loop for major-mode in major-modes do
        (let ((major-mode (intern (concat (symbol-name major-mode) "-mode")))
              (n (if (string-match "\\\\([^?]" regexp) 1 0)))
          (font-lock-add-keywords major-mode
                                  `((,regexp (0 (prog1 ()
                                                       (compose-region (match-beginning ,n)
                                                                       (match-end ,n)
                                                                       ,glyph)))))))))))

(set-pretty-patterns
 '((?λ ("\\<lambda\\>" lisp lisp-interaction emacs-lisp scheme))
   (?φ ("\\<nil\\>" lisp lisp-interaction emacs-lisp scheme))
   (?λ ("\\<function\\>" js2))))