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;;;; ;;;;;;;;
;;;; Even Fibonacci numbers
;;;; Problem 2
;;;;
;;;; Each new term in the Fibonacci sequence is generated by adding the previous two terms.
;;;; By starting with 1 and 2, the first 10 terms will be:
;;;; 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
;;;; By considering the terms in the Fibonacci sequence whose values do not exceed four million,
;;;; find the sum of the even-valued terms.
;;;; ;;;;;;;;
(defun sum (xs)
(if (null xs)
0
(+ (car xs) (sum (cdr xs)))))
(defun fib (x y)
(when (> x 4000000) (return-from fib ()))
(cons x (fib y (+ x y))))
(defvar *fibs-below-4m* (fib 1 2))
(setf *fibs-below-4m* (remove-if-not #'evenp *fibs-below-4m*))
(print (sum *fibs-below-4m*))
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