new example: unu in retro

FossilOrigin-Name: 5edd4a53544b383fed144cbb40f99ffc9fca62bc7fd6b6fc761a4bfbdddea563

example/retro-unu.forth

@@ -0,0 +1,22 @@
+#!/usr/bin/env retro
+
+~~~
+{{
+  'Fenced var
+  :toggle-fence @Fenced not !Fenced ;
+  :fenced? (-f) @Fenced ;
+  :handle-line (s-)
+    fenced? [ over call ] [ drop ] choose ;
+---reveal---
+  :unu (sq-)
+    swap [ dup '~~~ s:eq?
+           [ drop toggle-fence ]
+           [ handle-line       ] choose
+         ] file:for-each-line drop ;
+}}
+~~~
+
+~~~
+#0 sys:argv [ s:put nl ] unu
+~~~
+