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<H2>Refute 2013</H2>
Jasmin C. Blanchette<sup>1</sup>, Tjark Weber<sup>2</sup><BR>
<sup>1</sup>Technische Universit&auml;t M&uuml;nchen, Germany <BR>
<sup>2</sup>Uppsala Universitet, Sweden <BR>

<H3>Architecture</H3>

Refute [<A HREF="#References">Web08</A>] is an open source counterexample 
generator for Isabelle/HOL [<A HREF="#References">NPW13</A>] based on a
SAT solver, and Nitpick's [<A HREF="#References">BN10</A>] precursor.

<H3>Strategies</H3>

<p>
Refute employs a SAT solver to find a finite model of the negated conjecture.
The translation from HOL to propositional logic is parameterized by the
cardinalities of the atomic types occurring in the conjecture. Refute enumerates
the possible cardinalities for each atomic type. If a formula has a finite
counterexample, the tool eventually finds it, unless it runs out of resources.

<H3>Implementation</H3>

<p>
Refute, like most of Isabelle/HOL, is written in Standard ML. Unlike Isabelle
itself, which adheres to the LCF small-kernel discipline, Refute does not
certify its results and must be trusted.
<P>
Refute is available as part of Isabelle/HOL for all major platforms under a
BSD-style license from
<PRE>
    <A HREF="http:////www.cl.cam.ac.uk/research/hvg/Isabelle/">http://www.cl.cam.ac.uk/research/hvg/Isabelle</A></PRE>

<H3>Expected Competition Performance</H3>

We expect that Refute will solve about 75% of the problems solved by Nitpick in
the TNT category, and perhaps a few problems that Nitpick cannot solve.

<H3>References</H3>
<DL>
<DT> BN10
<DD> Blanchette J. C., Nipkow T. (2010),
     <STRONG>Nitpick: A Counterexample Generator for Higher-Order Logic Based on a Relational Model Finder</STRONG>,
     ITP 2010, <EM>LNCS</EM> 6172, pp. 131&ndash;146, Springer.
<DT> NPW13
<DD> Nipkow T., Paulson L. C., Wenzel M. (2013),
     <STRONG>Isabelle/HOL: A Proof Assistant for Higher-Order Logic</STRONG>,
     <A HREF="http://www.cl.cam.ac.uk/research/hvg/Isabelle/dist/Isabelle/doc/tutorial.pdf">http://www.cl.cam.ac.uk/research/hvg/Isabelle/dist/Isabelle/doc/tutorial.pdf</a>.
<DT> Web08
<DD> Weber T. (2008),
     <STRONG>SAT-based Finite Model Generation for Higher-Order Logic</STRONG>, Ph.D. thesis.
</DL>
<P>

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