;; Library: simple implementation of sets.
;; Put this in the same directory as "lc.scm".
(module sets mzscheme
  
  (provide empty-set
           singleton-set
           set-union
           set-minus
           is-in-set?)
  
  (define empty-set (make-hash-table))
  
  (define (singleton-set v)
    (let ([h (make-hash-table)])
      (hash-table-put! h v #t)
      h))
  
  (define (set-union s1 s2)
    (let ([h (hash-table-copy s1)])
      (hash-table-for-each
       s2
       (lambda (k v)
         (hash-table-put! h k #t)))
      h))
  
  (define (set-minus s1 x)
    (if (hash-table-get s1 x (lambda () #f))
        (let ([h (hash-table-copy s1)])
          (hash-table-remove! h x)
          h)
        s1))
  
  (define (is-in-set? x s1)
    (hash-table-get s1 x (lambda () #f))))