2017-03-19: added cartesian sets

added cartesian sets

diff --git a/examples/cartesian-sets.w b/examples/cartesian-sets.w
new file mode 100755
--- /dev/null
+++ b/examples/cartesian-sets.w
@@ -0,0 +1,39 @@
+#!/usr/bin/env sh
+# -*- wisp -*-
+exec guile -L $(dirname $(dirname $(realpath "$0"))) --language=wisp -e '(@@ (examples cartesian-sets) main)' -s "$0" "$@"
+; !#
+;; Implementation of the cartesian product over a list of lists, building on http://stackoverflow.com/a/20591545/7666
+
+define-module : examples cartesian-sets
+   . #:export : cartesian-product
+
+import : srfi srfi-1
+
+;; original
+define : cartesian-product-lambda . lists
+  fold-right 
+      lambda : xs ys
+          append-map (lambda (x)
+                      (map (lambda (y)
+                             (cons x y))
+                           ys))
+                    . xs
+      . '(())
+      . lists
+
+
+;; easier to understand
+define : cartesian-product . lists
+    define : product-of-two xs ys
+         define : cons-on-each-ys x
+             map : lambda (y) (cons x y)
+                 . ys
+         append-map cons-on-each-ys
+                  . xs
+    fold-right product-of-two '(()) lists
+
+define : main args
+    write : cartesian-product-lambda '(1 2) '(3 4) '(5 6)
+    newline
+    write : cartesian-product '(1 2) '(3 4) '(5 6)
+    newline