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<title>Concept Expr</title>
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<div class="refentry">
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<a name="Expr"></a><div class="titlepage"></div>
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<div class="refnamediv">
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<h2><span class="refentrytitle">Concept Expr</span></h2>
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<p>Expr</p>
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</div>
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<div class="refsect1">
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<a name="id-1.3.34.5.61.3"></a><h2>Description</h2>
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<p>
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An Expr represents a tagged node in an expression tree.
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The children of the Expr must themselves satisfy the
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Expr concept. The Expr has an arity representing the
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number of children. If the number of children is zero,
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the Expr also has a value. An Expr also has an associated
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<a class="link" href="Domain.html" title="Concept Domain">Domain</a>.
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</p>
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</div>
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<div class="refsect1">
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<a name="id-1.3.34.5.61.4"></a><h2>Associated types</h2>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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<p><span class="bold"><strong>proto_tag</strong></span></p>
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<pre class="literallayout">Expr::proto_tag</pre>
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<p>
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</p>
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<p>The tag type of the Expr.</p>
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<p>
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</p>
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</li>
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<li class="listitem">
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<p><span class="bold"><strong>proto_args</strong></span></p>
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<pre class="literallayout">Expr::proto_args</pre>
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<p>
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</p>
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<p>A typelist representing either the types of the child nodes, or,
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if the arity of the Expr is 0, of the value of the terminal.</p>
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<p>
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</p>
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</li>
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<li class="listitem">
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<p><span class="bold"><strong>proto_arity</strong></span></p>
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<pre class="literallayout">Expr::proto_arity</pre>
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<p>
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</p>
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<p>
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The arity (number of child nodes) of the Expr.
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<code class="computeroutput">proto_arity</code> is an MPL Integral Constant.
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</p>
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<p>
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</p>
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</li>
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<li class="listitem">
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<p><span class="bold"><strong>proto_grammar</strong></span></p>
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<pre class="literallayout">Expr::proto_grammar</pre>
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<p>
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</p>
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<p>
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A typedef for an instantiation of
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<code class="computeroutput"><a class="link" href="boost/proto/basic_expr.html" title="Struct template basic_expr">
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proto::basic_expr<>
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</a></code>
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that is equivalent to Expr. Expression types are equivalent if they have the
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same <code class="computeroutput">proto_tag</code>, <code class="computeroutput">proto_args</code>, and <code class="computeroutput">proto_arity</code>.
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</p>
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<p>
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</p>
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</li>
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<li class="listitem">
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<p><span class="bold"><strong>proto_base_expr</strong></span></p>
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<pre class="literallayout">Expr::proto_base_expr</pre>
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<p>
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</p>
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<p>
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A typedef for an instantiation of
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<code class="computeroutput"><a class="link" href="boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code> or
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<code class="computeroutput"><a class="link" href="boost/proto/basic_expr.html" title="Struct template basic_expr">proto::basic_expr<></a></code>
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that is equivalent to Expr. Expression types are equivalent if they have the
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same <code class="computeroutput">proto_tag</code>, <code class="computeroutput">proto_args</code>, and <code class="computeroutput">proto_arity</code>.
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</p>
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<p>
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</p>
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</li>
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<li class="listitem">
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<p><span class="bold"><strong>proto_derived_expr</strong></span></p>
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<pre class="literallayout">Expr::proto_derived_expr</pre>
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<p>
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</p>
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<p>
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A typedef for <code class="computeroutput">Expr</code>.
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</p>
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<p>
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</p>
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</li>
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<li class="listitem">
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<p><span class="bold"><strong>proto_domain</strong></span></p>
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<pre class="literallayout">Expr::proto_domain</pre>
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<p>
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</p>
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<p>
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The Domain of the Expr. <code class="computeroutput">proto_domain</code>
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models <a class="link" href="Domain.html" title="Concept Domain">Domain</a>.
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</p>
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<p>
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</p>
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</li>
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<li class="listitem">
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<p><span class="bold"><strong>proto_childN</strong></span></p>
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<pre class="literallayout">Expr::proto_childN</pre>
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<p>
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</p>
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<p>The type of the Nth child of Expr. Requires
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<code class="computeroutput">0 == N::value || N::value < proto_arity::value</code></p>
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<p>
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</p>
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</li>
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</ul></div>
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</div>
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<div class="refsect1">
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<a name="id-1.3.34.5.61.5"></a><h2>Notation</h2>
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<div class="variablelist"><dl class="variablelist">
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<dt><span class="term">Expr</span></dt>
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<dd>A type playing the role of expession-type in the <a class="link" href="Expr.html" title="Concept Expr">Expr</a> concept.</dd>
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<dt><span class="term">Tag</span></dt>
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<dd>A type playing the role of tag-type in the <a class="link" href="Expr.html" title="Concept Expr">Expr</a> concept.</dd>
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<dt><span class="term">Domain</span></dt>
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<dd>A type playing the role of domain-type in the <a class="link" href="Expr.html" title="Concept Expr">Expr</a> concept.</dd>
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<dt><span class="term">N</span></dt>
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<dd>A type playing the role of mpl-integral-constant-type in the <a class="link" href="Expr.html" title="Concept Expr">Expr</a> concept.</dd>
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<dt><span class="term"><code class="varname">e</code></span></dt>
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<dd>Object of type Expr</dd>
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</dl></div>
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</div>
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<div class="refsect1">
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<a name="id-1.3.34.5.61.6"></a><h2>Valid expressions</h2>
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<div class="informaltable"><table class="table">
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<colgroup>
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<col>
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<col>
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<col>
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<col>
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</colgroup>
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<thead><tr>
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<th>Name</th>
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<th>Expression</th>
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<th>Type</th>
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<th>Semantics</th>
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</tr></thead>
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<tbody>
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<tr>
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<td><p>Get N-th Child</p></td>
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<td><p>boost::proto::child< N >(e)</p></td>
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<td><p><span class="type">proto_childN</span></p></td>
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<td><p>Extracts the Nth child from this Expr.
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Requires <code class="computeroutput">N::value < proto_arity::value</code>.</p></td>
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</tr>
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<tr>
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<td><p>Get Terminal Value</p></td>
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<td><p>boost::proto::value(e)</p></td>
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<td><p><span class="type">proto_child0</span></p></td>
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<td><p>
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Extracts the value from a terminal Expr.
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Requires <code class="computeroutput">0 == proto_arity::value</code>.
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</p></td>
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</tr>
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<tr>
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<td><p>Get Base</p></td>
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<td><p>e.proto_base()</p></td>
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<td><p><span class="type">proto_base_expr</span></p></td>
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<td><p>
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Returns an object of type
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<code class="computeroutput"><a class="link" href="boost/proto/expr.html" title="Struct template expr">proto::expr<></a></code> or
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<code class="computeroutput"><a class="link" href="boost/proto/basic_expr.html" title="Struct template basic_expr">proto::basic_expr<></a></code>
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that is equivalent to <code class="computeroutput">e</code>.
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</p></td>
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</tr>
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</tbody>
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</table></div>
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</div>
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<div class="refsect1">
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<a name="id-1.3.34.5.61.7"></a><h2>Models</h2>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><span class="simplelist"><span class="type">boost::proto::literal< int ></span></span></li></ul></div>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2008 Eric Niebler<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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</p>
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</div></td>
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</tr></table>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="Domain.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="proto/reference.html"><img src="../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="ObjectTransform.html"><img src="../../doc/src/images/next.png" alt="Next"></a>
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