Extended the notion of letter and digit, such that now one may use greek,
gothic, euler, or calligraphic letters as normal letters.
(* ID: $Id$
To update:
cp /home/lcp/isabelle/Repos/HOL/Auth/{Message.thy,Message_lemmas.ML,Event.thy,Event_lemmas.ML,Public.thy,Public_lemmas.ML,NS_Public.thy} .
*)
use_thy "NS_Public";