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; Scheme 9 from Empty Space, Function Library
; By Nils M Holm, 2009
; Placed in the Public Domain
;
; (intersection list ...) ==> list
;
; Compute the intersection of a number of sets.
;
; Example: (intersection '(v w x) '(w x y) '(x y z)) ==> (x)
(define (intersection . a*)
(letrec
((intersection3 (lambda (a b r)
(cond ((null? a)
(reverse! r))
((member (car a) b)
(intersection3 (cdr a) b (cons (car a) r)))
(else
(intersection3 (cdr a) b r))))))
(if (null? a*)
a*
(fold-left (lambda (a b)
(intersection3 a b '()))
(car a*)
(cdr a*)))))
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