(define-library (csc list)
  (export
    intercalate
    revappend
    split-at
    take)
  (import (scheme base)
          (only (csc match)
            match))
  (begin


    (define (take n xs)
      (let loop ((n n)
                 (xs xs)
                 (acc '()))
        (if (or (not (positive? n)) (null? xs))
            (reverse acc)
          (loop (- n 1) (cdr xs) (cons (car xs) acc)))))


    (define (split-at n xs)
      (let loop ((n n)
                 (xs xs)
                 (acc '()))
        (if (or (not (positive? n)) (null? xs))
            (values (reverse acc) xs)
          (loop (- n 1) (cdr xs) (cons (car xs) acc)))))


    (define (revappend a b)
      (let loop ((xs a)
                 (acc b))
        (if (null? xs)
            acc
          (loop (cdr xs) (cons (car xs) acc)))))


    (define (intercalate x l)
      (match l
        ('() '())
        ((_) l)
        ((head . tail) (cons head (cons x (intercalate x tail))))))))