member?, let/cc

こういうのもありかな。

	(define (member? a lat)
	(let/cc skip
	(fold (lambda (e acc)
	(if (eq? e a)
	(skip #t)
	acc))
	#f lat)))
	(member? 1 '(a b c))
	; -> #f
	(member? 'c '(a b c d e))
	; -> #t

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