(define shift?
  (lambda (expr-stack next-token)
    (let ((stack-top (top-minus expr-stack 0)))
      (cond ((or (operator? stack-top)
                 (member stack-top '($ lparen)))
             (or (number? next-token)
                 (equal? next-token 'lparen)))
            ((number? stack-top)
             (let ((stack-second (top-minus expr-stack 1)))
               (cond ((operator? next-token)
                      (or (not (operator? stack-second))
                          (lower-precedence? stack-second
                                             next-token)))
                     ((equal? next-token 'rparen)
                      (equal? stack-second 'lparen))
                     (else #F))))
            (else #F)))))