rot13 demo from dan

demos/all.factor

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+REQUIRES: demos/color-picker demos/haar demos/lcd demos/nehe
+demos/numbers-game demos/hello-world demos/mslug-talk demos/rot13 ;
+
+PROVIDE: demos/all ;

demos/rot13.factor

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+! Copyright (C) 2006 Daniel Ehrenberg
+! See http://factorcode.org/license.txt for BSD license.
+IN: rot13
+USING: kernel math sequences strings ;
+
+: rotate ( ch base -- ch ) tuck - 13 + 26 mod + ;
+
+: rot-letter ( ch -- ch )
+    {
+        { [ dup letter? ] [ CHAR: a rotate ] }
+        { [ dup LETTER? ] [ CHAR: A rotate ] }
+        { [ t ] [ ] }
+    } cond ;
+
+: rot13 ( string -- string ) [ rot-letter ] map ;