; utils

(define fold-left
  (lambda (lst op e)
    (if (null? lst)
        e
        (fold-left (cdr lst) op (op e (car lst)))
        )
    )
  )

(define fold-right
  (lambda (lst op e)
    (if (null? lst)
        e
        (op (car lst) (fold-right (cdr lst) op e))
        )
    )
  )

(define zip
  (lambda (lst1 lst2)
    (map list lst1 lst2)
    )
  )

(define unzip
  (lambda (pairs)
    (list (map car pairs)
          (map cadr pairs))
    )
  )

(define filter
  (lambda (p lst)
    (if (null? lst)
        '()
        (if (p (car lst))
            (cons (car lst) (filter p (cdr lst)))
            (filter p (cdr lst))
            )
        )
    )
  )

(define displayln
  (lambda (x)
    (display x) (display "\n")
    )
  )