testest

;;TEST

;;1
(defun triangle-center (tri)
  (point (/ (+ (point-x (v1 tri)) (point-x (v2 tri)) (point-x (v3 tri))) 3)
         (/ (+ (point-y (v1 tri)) (point-y (v2 tri)) (point-y (v3 tri))) 3)))

;;2

(defun sum-digits (x)
  (if (< x 10)
      x
    (+ (rem x 10) (sum-digits (floor x 10)))))

;;3
;;(let ((last (cons 'B 'C))) (cons last (cons last 'A)))

;;4

(defun even-elements (list)
  (cond ((null list) nil)
         ((evenp (car list)) (cons (car list) (even-elements (cdr list))))
         (t (even-elements (cdr list)))))

;;5

(defun my-set-intersection-2 (a b)
  (cond ((null a) nil)
        ((null b) nil)
        ((contains b (car a)) (cons (car a) (my-set-intersection-2 (cdr a) b)))
        (t (my-set-intersection-2 (cdr a) b))))


(defun contains (list a)
  (cond ((null list) nil)
        ((= (car list) a) t)
        ((> (car list) a) nil)
        (t (contains (cdr list) a))))

;;6

(defun my-set-intersection (a &rest rest)
  (cond ((null rest) a)
        (t (apply #'my-set-intersection (my-set-intersection-2 a (car rest)) (cdr rest)))))

;;7
(defun seq-interleave (a b)
  (lambda (x) (if (evenp x) (funcall a (/ x 2)) (funcall b (floor (/ x 2)) ))))

(defun find-path (tree n)
  (cond ((null tree) nil)
        ((eql (car tree) n) (list (car tree)))
        ((null (cdr tree)) nil)
        (t (let ((next (find-path-children (cdr tree) n)))
          (if (eql next nil) nil (cons (car tree) next))))))


(defun find-path-children (children n)
  (cond ((null children) nil)
        (t (let ((child (find-path (car children) n)))
             (if (eql child nil) (find-path-children (cdr children) n) child)))))