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-About Isabelle
-
-%body%
-<h2>What is Isabelle?</h2>
-
-Isabelle can be viewed from two main perspectives.  On the one hand it
-may serve as a generic framework for rapid prototyping of deductive
-systems.  On the other hand, major existing logics like
-<strong>Isabelle/HOL</strong> provide a theorem proving environment
-ready to use for sizable applications.
-
-
-<h3>Isabelle's Logics</h3>
-
-The Isabelle distribution includes a large body of object logics and
-other examples (see the <a href="library/">Isabelle theory
-library</a>).
-
-<dl>
-
-<dt><a href="library/HOL/"><strong>Isabelle/HOL</strong></a><dd> is a
-version of classical higher-order logic resembling that of the <A
-HREF="http://www.cl.cam.ac.uk/Research/HVG/HOL/HOL.html">HOL
-System</A>.
-
-<dt><a href="library/HOLCF/"><strong>Isabelle/HOLCF</strong></a><dd>
-adds Scott's Logic for Computable Functions (domain theory) to HOL.
-
-<dt><a href="library/FOL/"><strong>Isabelle/FOL</strong></a><dd>
-provides basic classical and intuitionistic first-order logic.  It is
-polymorphic.
-
-<dt><a href="library/ZF/"><strong>Isabelle/ZF</strong></a><dd> offers
-a formulation of Zermelo-Fraenkel set theory on top of FOL.
-
-</dl>
-
-<p>
-
-Isabelle/HOL is currently the best developed object logic, including
-an extensive library of (concrete) mathematics, and various packages
-for advanced definitional concepts (like (co-)inductive sets and
-types, well-founded recursion etc.).  The distribution also includes
-some large applications, for example correctness proofs of
-cryptographic protocols (<a href="library/HOL/Auth/">HOL/Auth</a>) or
-communication protocols (<a href="library/HOLCF/IOA/">HOLCF/IOA</a>).
-
-<p>
-
-Isabelle/ZF provides another starting point for applications, with a
-slightly less developed library.  Its definitional packages are
-similar to those of Isabelle/HOL.  Untyped ZF provides more advanced
-constructions for sets than simply-typed HOL.
-
-<p>
-
-There are a few minor object logics that may serve as further
-examples: <a href="library/CTT/">CTT</a> is an extensional version of
-Martin-L&ouml;f's Type Theory, <a href="library/Cube/">Cube</a> is
-Barendregt's Lambda Cube.  There are also some sequent calculus
-examples under <a href="library/Sequents/">Sequents</a>, including
-modal and linear logics.  Again see the <a href="library/">Isabelle
-theory library</a> for other examples.
-
-
-<h3>Defining Logics</h3>
-
-Logics are not hard-wired into Isabelle, but formulated within
-Isabelle's meta logic: <strong>Isabelle/Pure</strong>.  There are
-quite a lot of syntactic and deductive tools available in generic
-Isabelle.  Thus defining new logics or extending existing ones
-basically works as follows:
-
-<ol>
-
-<li> declare concrete syntax (via mixfix grammar and syntax macros),
-
-<li> declare abstract syntax (as higher-order constants),
-
-<li> declare inference rules (as meta-logical propositions),
-
-<li> instantiate generic automatic proof tools (simplifier, classical
-tableau prover etc.),
-
-<li> manually code special proof procedures (via tacticals or
-hand-written ML).
-
-</ol>
-
-The first three steps above are fully declarative and involve no ML
-programming at all.  Thus one already gets a decent deductive
-environment based on primitive inferences (by employing the built-in
-mechanisms of Isabelle/Pure, in particular higher-order unification
-and resolution).
-
-For sizable applications some degree of automated reasoning is
-essential.  Instantiating existing tools like the classical tableau
-prover involves only minimal ML-based setup.  One may also write
-arbitrary proof procedures or even theory extension packages in ML,
-without breaching system soundness (Isabelle follows the well-known
-<em>LCF system approach</em> to achieve a secure system).