2015-03-11: add factorial example (beautiful code).

add factorial example (beautiful code).

diff --git a/examples/factorial.w b/examples/factorial.w
new file mode 100755
--- /dev/null
+++ b/examples/factorial.w
@@ -0,0 +1,16 @@
+#!/usr/bin/env sh
+exec guile -L $(dirname $(dirname $(realpath "$0"))) --language=wisp -e '(@@ (examples factorial) main)' -s "$0" "$@"
+; !#
+
+define-module : examples factorial
+              . #:export : factorial
+
+define : factorial n            ;   (define (factorial n)
+    if : zero? n                ;       (if (zero? n)
+       . n                      ; =>        n
+       * n : factorial {n - 1}  ;           (* n (factorial {n - 1}))))
+
+define : main args
+         display : factorial 5
+         newline
+