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))))

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

It is important that fail2 is called in the innermost if, so as to 
support expressions like:

(require (amb false true))