#lang racket

(define (subsets-of lst n)
  (cond
    [(null? lst) null] ; 0 ways of making anything out of empty set
    [(= n 0) (list null)] ; 1 way of making a 0 sized subset of something
    [(= n 1) (map list lst)] ; |lst| ways of making singletons
    [else
     (let ([head (car lst)]
           [tail (cdr lst)])
       (append
        ;; HEAD is a part of the subset, so the rest of the subset
        ;; must be an n-1 sized subset of TAIL
        (map (lambda (lst) (cons head lst))
             (subsets-of tail (- n 1)))
        ;; ... or HEAD is not part of the subset, so the subset is
        ;; some n sized subset of TAIL
        (subsets-of tail n)))]))

(define (powerset lst)
  (append* ; flatten list of n sized subsets to just get subsets
   (map (lambda (n)
          (subsets-of lst n)) ; get subset of size n
        (range 0 (+ 1 (length lst)))))) ; n from 0 to |lst|