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;;;; License: CC0-1.0
(define-module (net ricketyspace sicp two thirtytwo)
#:export (subsets))
(define (subsets s)
"All subsets of a Set X (P) is the union of all subsets of CDR of
Set X (Q) and a subset R derived from the union of CAR X with each of
the subsets in Q."
(if (null? s)
(list '())
(let ((rest (subsets (cdr s))))
(append rest (map (lambda (item) (cons (car s) item)) rest)))))
;;; Guile REPL
;;;
;;; scheme@(guile-user)> ,use (net ricketyspace sicp two thirtytwo)
;;; scheme@(guile-user)> (subsets '(1 2 3))
;;; $6 = (() (3) (2) (2 3) (1) (1 3) (1 2) (1 2 3))
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