Exercise 2.32: We can represent a set as a list
of distinct elements, and we can represent the set of all subsets of the set as
a list of lists.  For example, if the set is (1 2 3), then the set of
all subsets is (() (3) (2) (2 3) (1) (1 3) (1 2) (1 2 3)).  Complete the
following definition of a procedure that generates the set of subsets of a set
and give a clear explanation of why it works:


(define (subsets s)
  (if (null? s)
      (list nil)
      (let ((rest (subsets (cdr s))))
        (append rest (map ⟨??⟩ rest)))))