Exercise 4.5.  Scheme allows an additional syntax for cond  clauses, 
(<test> => <recipient>). If <test> evaluates to a true value, then 
<recipient> is evaluated. Its value must be a procedure of one argument; 
this procedure is then invoked on the value of the <test>, and the 
result is returned as the value of the cond expression. For example

(cond ((assoc 'b '((a 1) (b 2))) => cadr)
      (else false))

returns 2. Modify the handling of cond so that it supports this extended 
syntax.

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For the reasons similar to those in Exer. 4.4, this also cannot easily 
be implemented as a derived form using if.  If we do so, either the test 
will be evaluated more than once, or the generated expression will need 
to contain a new binding, which we cannot add arbitrarily.

Instead we can implement cond as a special form, and add a test and 
selectors for cond clauses containing arrows.

(define (eval exp env)
  (cond ...
        ((cond? exp) (eval-cond exp env))
        ...))

(define (cond-arrow-clause? clause) (eq? (cadr clause) '=>))
(define (cond-arrow-predicate clause) (car clause))
(define (cond-arrow-procedure-expr clause) (cddr clause))

(define (eval-cond exp env) (eval-cond-clauses (cond-clauses exp) env))

(define (eval-cond-clauses clauses env)
  (if (null? clauses)
      false
      (let ((first (car clauses))
            (rest (cdr clauses)))
        (cond ((cond-else-clause? first)
                 (if (null? rest)
                     (eval-sequence (cond-actions first) env)
                     (error "ELSE clause isn't last" clauses)))
              ((cond-arrow-clause? first)
                 (let ((result (eval (cond-arrow-predicate clause) env)))
                   (if result
                       (let ((procedure (eval (cond-arrow-procedure-expr clause) env)))
                         (apply procedure result))
                       (eval-cond-clauses rest env))))
              (else
                (if (cond-predicate first)
                    (eval (sequence->exp (cond-actions first)))
                    (eval-cond-clauses rest env)))))))