implement some list functions in built-in.scm

piclib/built-in.scm

@@ -21,3 +21,29 @@
 
 (define (list . args)
   args)
+
+(define (list? obj)
+  (if (null? obj)
+      #t
+      (if (pair? obj)
+	  (list? (cdr obj))
+	  #f)))
+
+(define (length list)
+  (if (null? list)
+      0
+      (+ 1 (length (cdr list)))))
+
+(define (append xs ys)
+  (if (null? xs)
+      ys
+      (cons (car xs)
+	    (append (cdr xs) ys))))
+
+(define (list-tail list k)
+  (if (zero? k)
+      list
+      (list-tail (cdr list) (- k 1))))
+
+(define (list-ref list k)
+  (car (list-tail list k)))