(defun setup-subscripts-display ()
  "Make numbers next to identifiers (like x0 or y4) appear like
subscripts. Uses `prettify-symbols-mode'."
  (setf prettify-symbols-unprettify-at-point 'right-edge)
  (mapc (lambda (x) (push x prettify-symbols-alist))
        '(("0" . "₀")
          ("1" . "₁")
          ("2" . "₂")
          ("3" . "₃")
          ("4" . "₄")
          ("5" . "₅")
          ("6" . "₆")
          ("7" . "₇")
          ("8" . "₈")
          ("9" . "₉")))
  (cl-macrolet ((save-start (&rest body) `(save-excursion (goto-char start) ,@body)))
    (setf prettify-symbols-compose-predicate
          (lambda (start end _match)
            (if (save-start (cl-digit-char-p (following-char)))
                (or
                 ;; #nλ subscripts
                 (and (save-start ; #
                       (loop while (cl-digit-char-p (following-char))
                             do (backward-char))
                       (eql (following-char) ?#))
                      (save-start ; λ
                       (forward-char)
                       (eql (following-char) ?λ)))
                 ;; regular subscribpts
                 (and (save-start ; excepted symbols
                       (not (member (sexp-at-point) '(prin1))))
                      (save-start ; on the left, ensure there's some letter
                       (loop while (cl-digit-char-p (following-char))
                             do (backward-char))
                       (string-match-p (rx letter) (char-to-string (following-char))))
                      (save-start ; on the right, there must be not other letters
                       (loop while (cl-digit-char-p (following-char))
                             do (forward-char))
                       (string-match-p (rx (or white ")" 10))
                                       (char-to-string (following-char))))))
              (prettify-symbols-default-compose-p start end _match)))))
  (prettify-symbols-mode))

(setup-subscripts-display)