Exercise 3.24.  In the table implementations above, the keys are tested 
for equality using equal? (called by assoc). This is not always the 
appropriate test. For instance, we might have a table with numeric keys 
in which we don't need an exact match to the number we're looking up, 
but only a number within some tolerance of it. Design a table 
constructor make-table that takes as an argument a same-key? procedure 
that will be used to test ``equality'' of keys. Make-table should return 
a dispatch procedure that can be used to access appropriate lookup and 
insert! procedures for a local table.

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(define (make-table same-key?)
  (define (assoc key records)
    (cond ((null? records) false)
          ((same-key? key (caar records)) (car records))
          (else (assoc key (cdr records)))))
  (let ((local-table (list '*table*)))
    ... as before ...