scheme - 如何在 DrRacket 中进行 powerset？

Question

我正在使用带有 DrRacket 列表缩写的起始语言，并且想要递归地创建一个 powerset，但无法弄清楚如何去做。我目前有这么多

(define
  (powerset aL)
  (cond
    [(empty? aL) (list)]

任何帮助都会很好。

Answer 1

            What's in a powerset? A set's subsets!             An empty set is any set's subset,            so powerset of empty set's not empty.             Its (only) element it is an empty set:

(define
  (powerset aL)
  (cond
    [(empty? aL) (list empty)]
    [else

            As for non-empty sets, there is a choice,            for each set's element, whether to be            or not to be included in subset            which is a member of a powerset. 
            We thus include both choices when combining             first element with smaller powerset,             that, which we get recursively applying             the same procedure to the rest of input:

       (combine (first aL)
                (powerset (rest aL)))]))

(define
  (combine a r)                      ; `r` for Recursive Result
  (cond
    [(empty? r)  empty]              ; nothing to combine `a` with
    [else
      (cons (cons a (first r))       ; Both add `a` and
          (cons (first r)            ;   don't add, to first subset in `r`
              (combine               ; and do the same
                    a                ;   with 
                    (rest r))))]))   ;   the rest of `r`

“没有答案，只有选择”。相当，
做出的选择，就是答案的构成。