clarified main ZF.thy / ZFC.thy, and avoid name clash with global HOL/Main.thy;

--HG--
rename : src/ZF/Main_ZF.thy => src/ZF/ZF.thy
rename : src/ZF/Main_ZFC.thy => src/ZF/ZFC.thy
rename : src/ZF/ZF.thy => src/ZF/ZF_Base.thy

clarified syntax;

isabelle update_cartouches -c -t;

isabelle update_cartouches;

modernized header;

tuned proofs;

recoded latin1 as utf8;
use textcomp for some text symbols where it appears appropriate;

removed old CVS Ids;
tuned headers;

tuned proofs -- avoid implicit prems;

tuned proofs;

migrated theory headers to new format

Extended the notion of letter and digit, such that now one may use greek,
gothic, euler, or calligraphic letters as normal letters.

conversion of AC branch to Isar

tuned document sources;

separation of the AC part of Main into Main_ZFC, plus a few new lemmas

Ntree and Brouwer converted and moved to ZF/Induct;