Что выполняет данный код?

(setq echo nil)
 
(LOOP (PRIN1 (QUOTE *)) (EVAL (READ)) ((NULL RDS)) )
 
(setq i1 0)
(setq j1 nil)
 
(defun RPDA ()
 (CLEAR-SCREEN)
 (PRINC "‚Введите арифметическое выражение: ")
 (SETQ rr (READSENT))
 (setq res (list '+))
 (NEXTSYMBOL)
 (EXPRESSION)
 (CLEAR-SCREEN)
 (PRINC "Арифметическая запись: ")
 (TERPRI)
 (PRINC rr)
 (TERPRI 2)
 (PRINC "Запись в ЛИСПЕ: ")
 (TERPRI)
 (PRINC res)
 (CLEAR-INPUT)
 (read-line)
 (system)
)
 
(defun NEXTSYMBOL ()
  (setq ScanSymbol (NTH i1 rr))
  (setq i1 (+ i1 1))
)
 
 
(defun WHILE1 ()
     (COND ((EQL ScanSymbol '+)
    (
     (NEXTSYMBOL)
     (ADDEND)
    ))
    (t (
     (setq res (append res (list (ASCII 40) '-)))
     (NEXTSYMBOL)
     (ADDEND)
     (setq res (append res (list (ASCII 41))))
    ))) ;COND
    (cond ((OR (EQL ScanSymbol '+) (EQL ScanSymbol '-))(WHILE1)))
)
 
 
(defun EXPRESSION ()
 (COND ((OR DIGIT (EQL ScanSymbol (ASCII 40)))
  (
(princ "Пре-слагаемое(Pre-Addend): ")
(princ res)
;(princ " SS: ")
;(princ ScanSymbol)
;(read-line)
   (ADDEND)
;(princ "Пост-слагаемое(Post Addend): ")
;(princ res)
;(princ " SS: ")
;(princ ScanSymbol)
;(read-line)
  (cond ((OR (EQL ScanSymbol '+) (EQL ScanSymbol '-))(WHILE1)))
  )
  )
 )
)
 
(defun WHILE2 ()
     (setq res1 (cons (NTH (- i 1) res) res1))
     (setq i (- i 1))
     (cond ((> i 0) (WHILE2)))
)
 
(defun WHILE3 ()
    (COND ((EQL ScanSymbol '*)
    (
     (NEXTSYMBOL)
     (FACTOR)
    ))
    (t (
     (setq res (append res (list (ASCII 40) '/)))
     (NEXTSYMBOL)
     (FACTOR)
     (setq res (append res (list (ASCII 41))))
    ))) ;COND
 (cond ((OR (EQL ScanSymbol '*) (EQL ScanSymbol '/))(WHILE3)))
)
 
 
(defun ADDEND ()
;(princ "ADDEND: ")
;(princ res)
;(princ " SS: ")
;(princ ScanSymbol)
;(read-line)
 (COND ((OR DIGIT (EQL ScanSymbol (ASCII 40)))
  (
   (setq j (length res))
   (setq j1 (cons j j1))
;(princ "pre-FACTOR: ")
;(princ res)
;(princ " SS: ")
;(princ ScanSymbol)
;(read-line)
   (FACTOR)
;(princ "post-FACTOR: ")
;(princ res)
;(princ " SS: ")
;(princ ScanSymbol)
;(read-line)
       (COND ((OR (EQL ScanSymbol '*) (EQL ScanSymbol '/))
   (
    (setq j (car j1))
    (setq j1 (cdr j1))
    (setq i j)
    (setq res1 nil)
    (cond ((> i 0) (WHILE2)))
    (setq res1 (append res1 (list (ascii 40) '*)))
    (setq res1 (append res1 (NTHCDR j res)))
    (setq res res1)
    (cond ((OR (EQL ScanSymbol '*) (EQL ScanSymbol '/))(WHILE3)))
 
    (setq res (append res (list (ASCII 41))))
   ))
   (t
   (
   (setq j1 (cdr j1))
   )))
 
 
  )))
)
 
 
(DEFUN FACTOR ()
;(princ "FACTOR: ")
;(princ res)
;(princ " SS: ")
;(princ ScanSymbol)
;(read-line)
 (cond ((DIGIT)
 (
  (setq res (append res (list ScanSymbol)))
  (NEXTSYMBOL)
 ))
       ((EQL ScanSymbol (ASCII 40))
  (
;        (princ "‘Є®Ў®зЄ  ¬ ©¤Ґ¬ : ")
;        (princ res)
;        (princ " SS: ")
;        (princ ScanSymbol)
;        (read-line)
    (setq res (append res (list (ascii 40) '+)))
    (NEXTSYMBOL)
    (EXPRESSION)
    (setq res (append res (list (ASCII 41))))
    (cond ((EQL ScanSymbol (ASCII 41)) ((NEXTSYMBOL))))
    
 
 ))
 )
)
 
 
 
 
(DEFUN DIGIT ()
(COND ( (or (eql ScanSymbol '+) (eql ScanSymbol '-) (eql ScanSymbol '*)
            (eql ScanSymbol '/) (eql ScanSymbol (ASCII 40))
            (eql ScanSymbol (ASCII 41))) nil) (t t))
)
 
 
 
(DEFUN READSENT (SENTENCE
    WORD )
  (LOOP
    (SETQ WORD (RATOM))
    ((MEMBER WORD TRMLIS)
      (REVERSE SENTENCE) )
    ( ((MEMBER WORD SEPLIS) 
        (PUSH (PACK* (POP SENTENCE) WORD (RATOM)) SENTENCE) )
 
      (PUSH WORD SENTENCE) ) ) )
 
(SETQ TRMLIS '("="))
(SETQ SEPLIS '("_"))
 
 
(RPDA (rds))