Exercise 4.54: If we had not realized that
require could be implemented as an ordinary procedure that uses
amb, to be defined by the user as part of a nondeterministic program, we
would have had to implement it as a special form.  This would require syntax
procedures


(define (require? exp) 
  (tagged-list? exp 'require))

(define (require-predicate exp) 
  (cadr exp))

and a new clause in the dispatch in analyze


((require? exp) (analyze-require exp))

as well the procedure analyze-require that handles require
expressions.  Complete the following definition of analyze-require.


(define (analyze-require exp)
  (let ((pproc (analyze 
                (require-predicate exp))))
    (lambda (env succeed fail)
      (pproc env
             (lambda (pred-value fail2)
               (if ⟨??⟩
                   ⟨??⟩
                   (succeed 'ok fail2)))
             fail))))