1 files changed, 15 insertions, 0 deletions

diff --git a/src/lisp/std.lisp b/src/lisp/std.lisp
index a256125..fe09a8a 100644
--- a/src/lisp/std.lisp
+++ b/src/lisp/std.lisp
@@ -72,3 +72,18 @@
           acc
           (rev-helper (cdr p) (cons (car p) acc))))
     (rev-helper ls '())))
+
+(define (append l1 l2)
+  (if (null? l1)
+      l2
+      (cons (car l1)
+            (append (cdr l1) l2))))
+
+(define (cross xs ys)
+  (if (or (null? xs)
+          (null? ys))
+      '()
+      (fold '()
+            append
+            (map (lambda (x)
+                   (map (lambda (y) (list x y)) ys)) xs))))