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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Set constraint</span></span>
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<p>In <a href="Mathematics" title="Mathematics">mathematics</a> and <a href="Theoretical_computer_science" title="Theoretical computer science">theoretical computer science</a>, a <b>set constraint</b> is an equation or an inequation between sets of <a href="Term_(logic)#Formal_definition" title="Term (logic)">terms</a>.
Similar to systems of (<a href="Inequation#Solving_inequations" title="Inequation">in</a>)<a href="Equation_solving" title="Equation solving">equations</a> between numbers, methods are studied for solving systems of set constraints.
Different approaches admit different operators (like "∪", "∩", "\", and function application)<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>note 1<span class="cite-bracket">]</span></a></sup> on sets and different (in)equation relations (like "=", "⊆", and "⊈") between set expressions.
</p><p>Systems of set constraints are useful to describe (in particular infinite) sets of <a href="Ground_term" class="mw-redirect" title="Ground term">ground terms</a>.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>note 2<span class="cite-bracket">]</span></a></sup>
They arise in program analysis, <a href="Abstract_interpretation" title="Abstract interpretation">abstract interpretation</a>, and <a href="Type_inference" title="Type inference">type inference</a>.
</p>
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<div class="mw-heading mw-heading2"><h2 id="Relation_to_regular_tree_grammars">Relation to regular tree grammars</h2></div>
<p>Each <a href="Regular_tree_grammar" title="Regular tree grammar">regular tree grammar</a> can be systematically transformed into a system of set inclusions such that its minimal solution corresponds to the tree language of the grammar.
</p><p>For example, the grammar (terminal and nonterminal symbols indicated by lower and upper case initials, respectively) with the rules
</p>
<dl><dd><table>
<tbody><tr>
<td><i>Bool</i><sub>G</sub></td>
<td>→ <i>false</i>
</td></tr>
<tr>
<td><i>Bool</i><sub>G</sub></td>
<td>→ <i>true</i>
</td></tr>
<tr>
<td><i>BList</i><sub>G</sub></td>
<td>→ <i>nil</i>
</td></tr>
<tr>
<td><i>BList</i><sub>G</sub></td>
<td>→ <i>cons</i>(<i>Bool</i><sub>G</sub>,<i>BList</i><sub>G</sub>)
</td></tr>
<tr>
<td><i>BList1</i><sub>G</sub></td>
<td>→ <i>cons</i>(<i>true</i>,<i>BList</i><sub>G</sub>)
</td></tr>
<tr>
<td><i>BList1</i><sub>G</sub></td>
<td>→ <i>cons</i>(<i>false</i>,<i>BList1</i><sub>G</sub>)
</td></tr></tbody></table></dd></dl>
<p>is transformed to the set inclusion system (constants and variables indicated by lower and upper case initials, respectively):
</p>
<dl><dd><table>
<tbody><tr>
<td><i>Bool</i><sub>S</sub></td>
<td>⊇ <i>false</i>
</td></tr>
<tr>
<td><i>Bool</i><sub>S</sub></td>
<td>⊇ <i>true</i>
</td></tr>
<tr>
<td><i>BList</i><sub>S</sub></td>
<td>⊇ <i>nil</i>
</td></tr>
<tr>
<td><i>BList</i><sub>S</sub></td>
<td>⊇ <i>cons</i>(<i>Bool</i><sub>S</sub>,<i>BList</i><sub>S</sub>)
</td></tr>
<tr>
<td><i>BList1</i><sub>S</sub></td>
<td>⊇ <i>cons</i>(<i>true</i>,<i>BList</i><sub>S</sub>)
</td></tr>
<tr>
<td><i>BList1</i><sub>S</sub></td>
<td>⊇ <i>cons</i>(<i>false</i>,<i>BList1</i><sub>S</sub>)
</td></tr></tbody></table></dd></dl>
<p>This system has a minimal solution, viz. ("<i>L</i>(<i>N</i>)" denoting the tree language corresponding to the nonterminal <i>N</i> in the above tree grammar):
</p>
<dl><dd><table>
<tbody><tr>
<td><i>Bool</i><sub>S</sub></td>
<td>= <i>L</i>(<i>Bool</i><sub>G</sub>)</td>
<td>= { <i>false</i>, <i>true</i> }
</td></tr>
<tr>
<td><i>BList</i><sub>S</sub></td>
<td>= <i>L</i>(<i>BList</i><sub>G</sub>)</td>
<td>= { <i>nil</i>, <i>cons</i>(<i>false</i>,<i>nil</i>), <i>cons</i>(<i>true</i>,<i>nil</i>), <i>cons</i>(<i>false</i>,<i>cons</i>(<i>false</i>,<i>nil</i>)), ... }
</td></tr>
<tr>
<td><i>BList1</i><sub>S</sub></td>
<td>= <i>L</i>(<i>BList1</i><sub>G</sub>)</td>
<td>= { <i>nil</i>, <i>cons</i>(<i>true</i>,<i>nil</i>), <i>cons</i>(<i>true</i>,<i>cons</i>(<i>false</i>,<i>nil</i>)),... }
</td></tr></tbody></table></dd></dl>
<p>The maximal solution of the system is trivial; it assigns the set of all terms to every variable.
</p>
<div class="mw-heading mw-heading2"><h2 id="Literature">Literature</h2></div>
<ul><li><style data-mw-deduplicate="TemplateStyles:r1238218222">
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</style><cite id="CITEREFAiken,_A.1995" class="citation techreport cs1">Aiken, A. (1995). <a rel="nofollow" class="external text" href="http://citeseer.uark.edu:8080/citeseerx/viewdoc/summary;jsessionid=01CAFC5839497EE6030F707B5B5C9CAA?doi=10.1.1.47.537"><i>Set Constraints: Results, Applications and Future Directions</i></a> (Technical report). Univ. Berkeley.</cite></li>
<li><cite id="CITEREFAiken,_A.,_Kozen,_D.,_Vardi,_M.,_Wimmers,_E.L.1993" class="citation techreport cs1">Aiken, A., Kozen, D., Vardi, M., Wimmers, E.L. (May 1993). <a rel="nofollow" class="external text" href="http://theory.stanford.edu/~aiken/publications/papers/csl93.ps"><i>The Complexity of Set Constraints</i></a> (Technical report). Computer Science Department, Cornell University. 93–1352.</cite><span class="cs1-maint citation-comment"><code class="cs1-code">{{cite tech report}}</code>: CS1 maint: multiple names: authors list (link)</span></li>
<li><cite id="CITEREFAiken,_A.,_Kozen,_D.,_Vardi,_M.,_Wimmers,_E.L.1994" class="citation book cs1">Aiken, A., Kozen, D., Vardi, M., Wimmers, E.L. (1994). "The Complexity of Set Constraints". <i>Computer Science Logic'93</i>. LNCS. Vol. 832. Springer. pp. <span class="nowrap">1–</span>17.</cite><span class="cs1-maint citation-comment"><code class="cs1-code">{{cite book}}</code>: CS1 maint: multiple names: authors list (link)</span></li>
<li><cite id="CITEREFAiken,_A.,_Wimmers,_E.L.1992" class="citation book cs1">Aiken, A., Wimmers, E.L. (1992). "Solving Systems of Set Constraints (Extended Abstract)". <i>Seventh Annual IEEE Symposium on Logic in Computer Science</i>. pp. <span class="nowrap">329–</span>340.</cite><span class="cs1-maint citation-comment"><code class="cs1-code">{{cite book}}</code>: CS1 maint: multiple names: authors list (link)</span></li>
<li><cite id="CITEREFBachmair,_Leo,_Ganzinger,_Harald,_Waldmann,_Uwe1992" class="citation techreport cs1">Bachmair, Leo, Ganzinger, Harald, Waldmann, Uwe (1992). <i>Set Constraints are the Monadic Class</i> (Technical report). Max-Planck-Institut für Informatik. p. 13. <a href="CiteSeerX_(identifier)" class="mw-redirect" title="CiteSeerX (identifier)">CiteSeerX</a> <span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.32.3739">10.1.1.32.3739</a></span>. MPI-I-92-240.</cite><span class="cs1-maint citation-comment"><code class="cs1-code">{{cite tech report}}</code>: CS1 maint: multiple names: authors list (link)</span></li>
<li><cite id="CITEREFBachmair,_Leo,_Ganzinger,_Harald,_Waldmann,_Uwe1993" class="citation book cs1">Bachmair, Leo, Ganzinger, Harald, Waldmann, Uwe (1993). "Set Constraints are the Monadic Class". <i>Eight Annual IEEE Symposium on Logic in Computer Science</i>. pp. <span class="nowrap">75–</span>83.</cite><span class="cs1-maint citation-comment"><code class="cs1-code">{{cite book}}</code>: CS1 maint: multiple names: authors list (link)</span></li>
<li><cite id="CITEREFCharatonik,_W.1994" class="citation book cs1">Charatonik, W. (Sep 1994). "Set Constraints in Some Equational Theories". <i>Proc. 1st Int. Conf. on Constraints in Computational Logics (CCL)</i>. LNCS. Vol. 845. Springer. pp. <span class="nowrap">304–</span>319.</cite></li>
<li><cite id="CITEREFCharatonikPodelski2002" class="citation journal cs1">Charatonik, Witold; Podelski, Andreas (2002). <a rel="nofollow" class="external text" href="https://doi.org/10.1006%2Finco.2001.2952">"Set Constraints with Intersection"</a>. <i>Information and Computation</i>. <b>179</b> (2): <span class="nowrap">213–</span>229. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://doi.org/10.1006%2Finco.2001.2952">10.1006/inco.2001.2952</a></span>.</cite></li>
<li><cite id="CITEREFCharatonik,_W.,_Podelski,_A.1998" class="citation book cs1">Charatonik, W., Podelski, A. (1998). <a href="Tobias_Nipkow" title="Tobias Nipkow">Tobias Nipkow</a> (ed.). <i>Co-definite Set Constraints</i>. LNCS 1379. Springer-Verlag. pp. <span class="nowrap">211–</span>225.</cite><span class="cs1-maint citation-comment"><code class="cs1-code">{{cite book}}</code>: CS1 maint: multiple names: authors list (link)</span></li>
<li><cite id="CITEREFCharatonik,_W.,_Talbot,_J.-M.2002" class="citation book cs1">Charatonik, W., Talbot, J.-M. (2002). Tison, S. (ed.). <i>Atomic Set Constraints with Projection</i>. LNCS 2378. Springer. pp. <span class="nowrap">311–</span>325.</cite><span class="cs1-maint citation-comment"><code class="cs1-code">{{cite book}}</code>: CS1 maint: multiple names: authors list (link)</span></li>
<li><cite id="CITEREFGilleron,_R.,_Tison,_S.,_Tommasi,_M.1993" class="citation book cs1">Gilleron, R., Tison, S., Tommasi, M. (1993). "Solving Systems of Set Constraints using Tree Automata". <i>10th Annual Symposium on Theoretical Aspects of Computer Science</i>. LNCS. Vol. 665. Springer. pp. <span class="nowrap">505–</span>514.</cite><span class="cs1-maint citation-comment"><code class="cs1-code">{{cite book}}</code>: CS1 maint: multiple names: authors list (link)</span></li>
<li><cite id="CITEREFHeintze,_N.,_Jaffar,_J.1990" class="citation book cs1">Heintze, N., Jaffar, J. (1990). "A Decision Procedure for a Class of Set Constraints (Extended Abstract)". <i>Fifth Annual IEEE Symposium on Logic in Computer Science</i>. pp. <span class="nowrap">42–</span>51.</cite><span class="cs1-maint citation-comment"><code class="cs1-code">{{cite book}}</code>: CS1 maint: multiple names: authors list (link)</span></li>
<li><cite id="CITEREFHeintze,_N.,_Jaffar,_J.1991" class="citation techreport cs1">Heintze, N., Jaffar, J. (Feb 1991). <i>A Decision Procedure for a Class of Set Constraints</i> (Technical report). School of Computer Science, Carnegie Mellon University.</cite><span class="cs1-maint citation-comment"><code class="cs1-code">{{cite tech report}}</code>: CS1 maint: multiple names: authors list (link)</span></li>
<li><cite id="CITEREFKozen,_D.1993" class="citation book cs1">Kozen, D. (1993). <a rel="nofollow" class="external text" href="http://www.cs.cornell.edu/~kozen/papers/lasc.pdf">"Logical Aspects of Set Constraints"</a> <span class="cs1-format">(PDF)</span>. <i>Computer Science Logic'93</i>. LNCS. Vol. 832. pp. <span class="nowrap">175–</span>188.</cite></li>
<li><cite id="CITEREFKozen,_D.1994" class="citation book cs1">Kozen, D. (1994). "Set Constraints and Logic Programming". <i>CCL</i>. LNCS. Vol. 845.</cite></li>
<li><cite id="CITEREFDexter_Kozen1998" class="citation journal cs1">Dexter Kozen (1998). <a rel="nofollow" class="external text" href="https://doi.org/10.1006%2Finco.1997.2694">"Set Constraints and Logic Programming"</a>. <i>Information and Computation</i>. <b>142</b>: <span class="nowrap">2–</span>25. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://doi.org/10.1006%2Finco.1997.2694">10.1006/inco.1997.2694</a></span>.</cite></li>
<li><cite id="CITEREFUribe,_T.E.1992" class="citation book cs1">Uribe, T.E. (1992). <a rel="nofollow" class="external text" href="http://theory.stanford.edu/~uribe/papers/unification.ps.Z">"Sorted Unification Using Set Constraints"</a>. <i>Proc. CADE–11</i>. LNCS. Vol. 607. pp. <span class="nowrap">163–</span>177.</cite></li></ul>
<div class="mw-heading mw-heading3"><h3 id="Literature_on_negative_constraints">Literature on negative constraints</h3></div>
<ul><li><cite id="CITEREFAiken,_A.,_Kozen,_D.,_Wimmers,_E.L.1993" class="citation techreport cs1">Aiken, A., Kozen, D., Wimmers, E.L. (Jun 1993). <a rel="nofollow" class="external text" href="http://theory.stanford.edu/~aiken/publications/papers/ic95.ps"><i>Decidability of Systems of Set Constraints with Negative Constraints</i></a> (Technical report). Computer Science Department, Cornell University. 93–1362.</cite><span class="cs1-maint citation-comment"><code class="cs1-code">{{cite tech report}}</code>: CS1 maint: multiple names: authors list (link)</span></li>
<li><cite id="CITEREFCharatonik,_W.,_Pacholski,_L.1994" class="citation book cs1">Charatonik, W., Pacholski, L. (Jul 1994). "Negative Set Constraints with Equality". <i>Ninth Annual IEEE Symposium on Logic in Computer Science</i>. pp. <span class="nowrap">128–</span>136.</cite><span class="cs1-maint citation-comment"><code class="cs1-code">{{cite book}}</code>: CS1 maint: multiple names: authors list (link)</span></li>
<li><cite id="CITEREFR._GilleronS._TisonM._Tommasi1993" class="citation book cs1">R. Gilleron; S. Tison; M. Tommasi (1993). "Solving Systems of Set Constraints with Negated Subset Relationships". <i>Proceedings of the 34th Symp. on Foundations of Computer Science</i>. pp. <span class="nowrap">372–</span>380.</cite></li>
<li><cite id="CITEREFGilleron,_R.,_Tison,_S.,_Tommasi,_M.1993" class="citation techreport cs1">Gilleron, R., Tison, S., Tommasi, M. (1993). <i>Solving Systems of Set Constraints with Negated Subset Relationships</i> (Technical report). Laboratoire d'Informatique Fondamentale de Lille. IT 247.</cite><span class="cs1-maint citation-comment"><code class="cs1-code">{{cite tech report}}</code>: CS1 maint: multiple names: authors list (link)</span></li>
<li><cite id="CITEREFStefansson,_K.1993" class="citation techreport cs1">Stefansson, K. (Aug 1993). <i>Systems of Set Constraints with Negative Constraints are NEXPTIME-Complete</i> (Technical report). Computer Science Department, Cornell University. 93–1380.</cite></li>
<li><cite id="CITEREFStefansson,_K.1994" class="citation book cs1">Stefansson, K. (1994). "Systems of Set Constraints with Negative Constraints are NEXPTIME-Complete". <i>Ninth Annual IEEE Symposium on Logic in Computer Science</i>. pp. <span class="nowrap">137–</span>141.</cite></li></ul>
<div class="mw-heading mw-heading2"><h2 id="Notes">Notes</h2></div>
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<div class="mw-references-wrap"><ol class="references">
<li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text">If <i>f</i> is an <i>n</i>-ary function symbol admitted in a term, then "<i>f</i>(<i>E</i><sub>1</sub>,...,<i>E</i><sub><i>n</i></sub>)" is a set expression denoting the set { <i>f</i>(<i>t</i><sub>1</sub>,...,<i>t</i><sub><i>n</i></sub>) : <i>t</i><sub>1</sub>∈<i>E</i><sub>1</sub> and ... and <i>t</i><sub><i>n</i></sub>∈<i>E</i><sub><i>n</i></sub> }, where <i>E</i><sub>1</sub>,...,<i>E</i><sub><i>n</i></sub> are set expressions in turn.</span>
</li>
<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text">This is similar to describing e.g. a <a href="Rational_number" title="Rational number">rational number</a> as a solution to an equation <i>a</i>⋅<i>x</i> + <i>b</i> = 0, with <a href="Integer" title="Integer">integer</a> coefficients <i>a</i>, <i>b</i>.</span>
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