;;;; ;;;;;;;;
;;;; 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*))