(define (equal? a b)
  (if (nil? a) (if (nil? b) 1 0)
    (if (atom a)
      (if (atom b) (= a b) 0)
      (if (atom b) 0 (if (equal? (car a) (car b)) (equal? (cdr a) (cdr b)) 0))
    )
  )
)

(define (empty? l)
    (equal? l (nil))
)

(define (not a)
    (if a 0 1)
)

(not 2)
(not 1)
(not 0)


(define (lhas list what)
  (if (empty? list) 0
  (if (equal? (car list) what) 1 (lhas (cdr list) what))
)
)

(define (onehas list l)
  (if (empty? list) 0
  (if (l (car list)) 1 (onehas (cdr list) l))
)
)

(define (appfront list what)
(cons what list)
  )

(define (pathslist acc paths from)
  (if (empty? paths)
      acc
      (if (equal? (car (car paths)) from)
          (pathslist (appfront acc (car(cdr (car paths)))) (cdr paths) from)
          (pathslist acc (cdr paths) from)
          )
  )
)

(equal? (pathslist () (quote((1 2))) 1) (quote(2)))


(define (filter p l)
(if (empty? l) (nil)
    (if (p (car l))
        (cons (car l) (filter p (cdr l)) )
        (filter p (cdr l))
        )
))

(filter (lambda (x) (if (> x 0) 1 0 )) (quote(-1 2 0 3 -3 1)))
(equal? (filter (lambda (x) (if (> (car x) 0) (if (> (car(cdr x)) 0) 1 0) 0 )) (quote((-1 2) (0 3) (-3 1) (1 1)))) (quote((1 1))))

(define (cities-path-impl seen paths from to)
  (if (equal? from to)
      1
      (onehas
             (filter (lambda (w) (not (lhas (appfront seen from) w))) (pathslist (nil) paths from))
             (lambda (f) (cities-path-impl (appfront seen from) paths f to))
       )
      )
  )

(define (cities-path? paths from to)
  (cities-path-impl (nil) paths from to)
)

(cities-path? (nil) 1 2)
(cities-path? (nil) 2 2)
(cities-path? (quote((1 2))) 1 2)
(cities-path? (quote((1 2))) 1 3)
(cities-path? (quote((1 2))) 2 1)
(cities-path? (quote((2 1))) 1 2)
(cities-path? (quote((1 2) (2 1))) 1 2)
(cities-path? (quote((1 2) (2 1))) 2 1)
(cities-path? (quote((1 2) (2 3) (3 4) (4 2))) 1 2)
(cities-path? (quote((1 2) (2 3) (3 4) (4 2))) 1 3)
(cities-path? (quote((1 2) (2 3) (3 4) (4 2))) 1 4)
(cities-path? (quote((1 2) (2 3) (3 4) (4 2))) 2 3)
(cities-path? (quote((1 2) (2 3) (3 4) (4 2))) 2 4)
(cities-path? (quote((4 2) (3 4) (2 3) (1 2))) 3 2)