added an intro lemma for freshness of products; set up
the simplifier so that it can deal with the compact and
long notation for freshness constraints (FIXME: it should
also be able to deal with the special case of freshness
of atoms)
%FIXME
%\chapter{Basic Concepts}\label{ch:basics}
%\section{The Isar proof language}
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%%% TeX-master: "isar-ref"
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