[isabelle] Inner Lexical Error

Hello all,

I've installed Isabelle on Ubuntu 8.10 and I use isabelle emacs to start
the system. I also turn X-symbol on in Proof General but when I compile
Message.thy(get it from the Isabelle Library 2009 :
http://isabelle.in.tum.de/library/HOL/Auth/NS_Shared.html or other theory
files using math symbols), I get "Inner lexical error" or "Inner Syntax
Error", "Failed to Parse Proposition" at commands which contain \<union> 
or existential quantifier(but it can parse \<in> symbol)?!!
What is wrong?

Any help would be great.

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