make-expression

;;; This is a slightly updated version of the code presented
;;; on Jan. 14. The updates include a correction to the
;;; call to VAL in the second case, and some reformatting.

(defun make-expression (english-desc)
"Returns a Lisp expression corresponding to
    the ENGLISH-DESC."
    (let (v)
        (cond
            ((setq v (match '(the product of (* x) times (* y))
                english-desc))
             (list '*
                   (make-expression (val 'x v))
                   (make-expression (val 'y v)) ) )
            ((setq v (match '(the (operator-name op) of (* x))
                english-desc))
             (list (val 'op v) (make-expression (val 'x v))) )
            ((setq v (match '(the (? x)) english-desc))
             (val 'x v) )
            ((setq v (match '((? x)) english-desc))
             (val 'x v) )
        ) ) )

(defun operator-name (name)
"Returns true if NAME is the name of an operator."
(member name '(square cube square-root negative)) )