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<title>Associative
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Sequence</title>
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<div class="section" lang="en">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="fusion.sequences.concepts.associative_sequence"></a><a href="associative_sequence.html" title="Associative
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Sequence">Associative
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Sequence</a></h4></div></div></div>
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<a name="fusion.sequences.concepts.associative_sequence.description"></a><h5>
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<a name="id475992"></a>
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<a href="associative_sequence.html#fusion.sequences.concepts.associative_sequence.description">Description</a>
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</h5>
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<p>
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An Associative Sequence allows efficient retrieval of elements based on
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keys. Like associative sequences in <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>,
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and unlike associative containers in <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>,
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Fusion associative sequences have no implied ordering relation. Instead,
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type identity is used to impose an equivalence relation on keys, and the
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order in which sequence elements are traversed during iteration is left
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unspecified. In addition, unlike <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>,
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Associative Sequences have mutable iterators. This is due to the fact that
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there is no associated ordering relation and the runtime value of the keys
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themselves do not have any effect on the associativity of the sequence.
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</p>
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<div class="variablelist">
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<p class="title"><b>Notation</b></p>
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<dl>
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<dt><span class="term"><tt class="computeroutput"><span class="identifier">s</span></tt></span></dt>
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<dd>
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An Associative Sequence
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</dd>
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<dt><span class="term"><tt class="computeroutput"><span class="identifier">S</span></tt></span></dt>
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<dd>
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An Associative Sequence type
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</dd>
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<dt><span class="term"><tt class="computeroutput"><span class="identifier">K</span></tt></span></dt>
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<dd>
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An arbitrary <span class="emphasis"><em>key</em></span> type
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</dd>
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<dt><span class="term"><tt class="computeroutput"><span class="identifier">o</span></tt></span></dt>
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<dd>
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An arbitrary object
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</dd>
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<dt><span class="term"><tt class="computeroutput"><span class="identifier">e</span></tt></span></dt>
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<dd>
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A Sequence element
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</dd>
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</dl>
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</div>
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<a name="fusion.sequences.concepts.associative_sequence.valid_expressions"></a><h5>
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<a name="id476156"></a>
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<a href="associative_sequence.html#fusion.sequences.concepts.associative_sequence.valid_expressions">Valid
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Expressions</a>
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</h5>
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<p>
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For any Associative Sequence the following expressions must be valid:
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</p>
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<div class="informaltable">
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<h4>
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<a name="id476184"></a>
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</h4>
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<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>Expression</th>
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<th>Return type</th>
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<th>Type Requirements</th>
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<th>Runtime
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Complexity</th>
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</tr></thead>
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<tbody>
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<tr>
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<td><tt class="computeroutput"><a href="../intrinsics/functions/has_key.html" title="has_key"><tt class="computeroutput"><span class="identifier">has_key</span></tt></a><span class="special"><</span><span class="identifier">K</span><span class="special">>(</span><span class="identifier">s</span><span class="special">)</span></tt></td>
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<td>MPL
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Boolean Constant. Convertible to bool.</td>
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<td> </td>
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<td>Constant</td>
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</tr>
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<tr>
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<td><tt class="computeroutput"><a href="../intrinsics/functions/at_key.html" title="at_key"><tt class="computeroutput"><span class="identifier">at_key</span></tt></a><span class="special"><</span><span class="identifier">K</span><span class="special">>(</span><span class="identifier">s</span><span class="special">)</span></tt></td>
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<td>Any
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type</td>
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<td> </td>
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<td>Constant</td>
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</tr>
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<tr>
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<td><tt class="computeroutput"><a href="../intrinsics/functions/at_key.html" title="at_key"><tt class="computeroutput"><span class="identifier">at_key</span></tt></a><span class="special"><</span><span class="identifier">K</span><span class="special">>(</span><span class="identifier">s</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">o</span></tt></td>
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<td>Any
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type</td>
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<td>
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<tt class="computeroutput"><span class="identifier">s</span></tt>
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is mutable and <tt class="computeroutput"><span class="identifier">e</span> <span class="special">=</span> <span class="identifier">o</span></tt>,
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where <tt class="computeroutput"><span class="identifier">e</span></tt> is the first
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element in the sequence, is a valid expression.</td>
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<td>Constant</td>
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</tr>
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</tbody>
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</table>
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</div>
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<a name="fusion.sequences.concepts.associative_sequence.result_type_expressions"></a><h5>
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<a name="id476459"></a>
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<a href="associative_sequence.html#fusion.sequences.concepts.associative_sequence.result_type_expressions">Result
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Type Expressions</a>
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</h5>
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<div class="informaltable">
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<h4>
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<a name="id476482"></a>
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</h4>
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<table class="table">
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<colgroup>
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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>Expression</th>
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<th>Compile Time Complexity</th>
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</tr></thead>
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<tbody>
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<tr>
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<td><tt class="computeroutput"><a href="../intrinsics/metafunctions/has_key.html" title="has_key"><tt class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">has_key</span></tt></a><span class="special"><</span><span class="identifier">S</span><span class="special">,</span> <span class="identifier">K</span><span class="special">>::</span><span class="identifier">type</span></tt></td>
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<td>Amortized constant
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time</td>
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</tr>
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<tr>
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<td><tt class="computeroutput"><a href="../intrinsics/metafunctions/at_key.html" title="at_key"><tt class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">at_key</span></tt></a><span class="special"><</span><span class="identifier">S</span><span class="special">,</span> <span class="identifier">K</span><span class="special">>::</span><span class="identifier">type</span></tt></td>
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<td>Amortized constant
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time</td>
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</tr>
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<tr>
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<td><tt class="computeroutput"><a href="../intrinsics/metafunctions/value_at_key.html" title="value_at_key"><tt class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_at_key</span></tt></a><span class="special"><</span><span class="identifier">S</span><span class="special">,</span> <span class="identifier">K</span><span class="special">>::</span><span class="identifier">type</span></tt></td>
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<td>Amortized constant
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time</td>
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</tr>
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</tbody>
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</table>
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</div>
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<p class="blurb">
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<span class="inlinemediaobject"><img src="../../../images/note.png" alt="note"></span> <tt class="computeroutput"><a href="../intrinsics/metafunctions/at_key.html" title="at_key"><tt class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">at_key</span></tt></a><span class="special"><</span><span class="identifier">S</span><span class="special">,</span> <span class="identifier">K</span><span class="special">></span></tt>
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returns the actual type returned by <tt class="computeroutput"><a href="../intrinsics/functions/at_key.html" title="at_key"><tt class="computeroutput"><span class="identifier">at_key</span></tt></a><span class="special"><</span><span class="identifier">K</span><span class="special">>(</span><span class="identifier">s</span><span class="special">)</span></tt>. In
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most cases, this is a reference. Hence, there is no way to know the exact
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element type using <tt class="computeroutput"><a href="../intrinsics/metafunctions/at_key.html" title="at_key"><tt class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">at_key</span></tt></a><span class="special"><</span><span class="identifier">S</span><span class="special">,</span> <span class="identifier">K</span><span class="special">></span></tt>.The
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element at <tt class="computeroutput"><span class="identifier">K</span></tt> may actually
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be a reference to begin with. For this purpose, you can use <tt class="computeroutput"><a href="../intrinsics/metafunctions/value_at_key.html" title="value_at_key"><tt class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_at_key</span></tt></a><span class="special"><</span><span class="identifier">S</span><span class="special">,</span> <span class="identifier">N</span><span class="special">></span></tt>.
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</p>
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<a name="fusion.sequences.concepts.associative_sequence.expression_semantics"></a><h5>
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<a name="id476991"></a>
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<a href="associative_sequence.html#fusion.sequences.concepts.associative_sequence.expression_semantics">Expression
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Semantics</a>
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</h5>
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<div class="informaltable">
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<h4>
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<a name="id477014"></a>
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</h4>
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<table class="table">
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<colgroup>
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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>Expression</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><tt class="computeroutput"><a href="../intrinsics/functions/has_key.html" title="has_key"><tt class="computeroutput"><span class="identifier">has_key</span></tt></a><span class="special"><</span><span class="identifier">K</span><span class="special">>(</span><span class="identifier">s</span><span class="special">)</span></tt></td>
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<td>A
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boolean Integral Constant <tt class="computeroutput"><span class="identifier">c</span></tt>
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such that <tt class="computeroutput"><span class="identifier">c</span><span class="special">::</span><span class="identifier">value</span> <span class="special">==</span> <span class="keyword">true</span></tt> if and only if there is one or
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more elements with the key <tt class="computeroutput"><span class="identifier">k</span></tt>
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in <tt class="computeroutput"><span class="identifier">s</span></tt>; see <a href="../intrinsics/functions/has_key.html" title="has_key"><tt class="computeroutput"><span class="identifier">has_key</span></tt></a>.</td>
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</tr>
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<tr>
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<td><tt class="computeroutput"><a href="../intrinsics/functions/at_key.html" title="at_key"><tt class="computeroutput"><span class="identifier">at_key</span></tt></a><span class="special"><</span><span class="identifier">K</span><span class="special">>(</span><span class="identifier">s</span><span class="special">)</span></tt></td>
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<td>The
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element associated with the key <tt class="computeroutput"><span class="identifier">K</span></tt>
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in the sequence <tt class="computeroutput"><span class="identifier">s</span></tt>;
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see <a href="../intrinsics/functions/at.html" title="at"><tt class="computeroutput"><span class="identifier">at</span></tt></a>.</td>
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</tr>
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</tbody>
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</table>
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</div>
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<a name="fusion.sequences.concepts.associative_sequence.models"></a><h5>
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<a name="id477276"></a>
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<a href="associative_sequence.html#fusion.sequences.concepts.associative_sequence.models">Models</a>
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</h5>
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<div class="itemizedlist"><ul type="disc">
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<li><a href="../containers/set.html" title="set"><tt class="computeroutput"><span class="identifier">set</span></tt></a></li>
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<li><a href="../containers/map.html" title="map"><tt class="computeroutput"><span class="identifier">map</span></tt></a></li>
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</ul></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"><small>Copyright © 2001-2005 Joel de Guzman, Dan Marsden</small></td>
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</tr></table>
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