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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<link rel="stylesheet" type="text/css" href="../boost.css">
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<title>Boost.Python - <wrapper.hpp></title>
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<table border="0" cellpadding="7" cellspacing="0" width="100%" summary=
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"header">
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<tr>
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<td valign="top" width="300">
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<h3><a href="../../../../index.htm"><img height="86" width="277" alt=
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"C++ Boost" src="../../../../boost.png" border="0"></a></h3>
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<td valign="top">
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<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
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<h2 align="center">Header <wrapper.hpp></h2>
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</table>
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<hr>
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<h2>Contents</h2>
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<dl class="page-index">
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<dt><a href="#introduction">Introduction</a></dt>
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<dt><a href="#classes">Classes</a></dt>
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<dd>
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<dl class="page-index">
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<dt><a href="#override-spec">Class template
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<code>override</code></a></dt>
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<dd>
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<dl class="page-index">
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<dt><a href="#override-spec-synopsis">Class
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<code>override</code> synopsis</a></dt>
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<dt><a href="#override-spec-observers">Class
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<code>override</code> observer functions</a></dt>
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</dl>
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</dd>
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<dt><a href="#wrapper-spec">Class template
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<code>wrapper</code></a></dt>
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<dd>
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<dl class="page-index">
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<dt><a href="#wrapper-spec-synopsis">Class <code>wrapper</code>
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synopsis</a></dt>
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<dt><a href="#wrapper-spec-observers">Class
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<code>wrapper</code> observer functions</a></dt>
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</dl>
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</dd>
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</dl>
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</dd>
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<dt><a href="#examples">Example(s)</a></dt>
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</dl>
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<hr>
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<h2><a name="introduction"></a>Introduction</h2>
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<p>To wrap a class <code>T</code> such that its virtual functions can be
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"overridden in Python"—so that the corresponding method of a Python
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derived class will be called when the virtual function is invoked from
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C++—you must create a C++ wrapper class derived from ``T`` that
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overrides those virtual functions so that they call into Python. This
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header contains classes that can be used to make that job easier.</p>
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<h2><a name="classes"></a>Classes</h2>
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<h3><a name="override-spec"></a>Class <code>override</code></h3>
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<p>Encapsulates a Python override of a C++ virtual function. An
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<code>override</code> object either holds a callable Python object or
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<code>None</code>.</p>
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<h4><a name="override-spec-synopsis"></a>Class <code>override</code>
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synopsis</h4>
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<pre>
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namespace boost
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{
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class override : object
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{
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public:
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<i>unspecified</i> operator() const;
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template <class A0>
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<i>unspecified</i> operator(A0) const;
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template <class A0, class A1>
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<i>unspecified</i> operator(A0, A1) const;
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...
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template <class A0, class A1, ...class A<i>n</i>>
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<i>unspecified</i> operator(A0, A1, ...A<i>n</i>) const;
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};
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};
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</pre>
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<h4><a name="override-spec-observers"></a>Class <code>override</code>
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observer functions</h4>
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<pre>
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<i>unspecified</i> operator() const;
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template <class A0>
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<i>unspecified</i> operator(A0) const;
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template <class A0, class A1>
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<i>unspecified</i> operator(A0, A1) const;
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...
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template <class A0, class A1, ...class A<i>n</i>>
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<i>unspecified</i> operator(A0, A1, ...A<i>n</i>) const;
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</pre>
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<dl class="function-semantics">
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<dt><b>Effects:</b> If <code>*this</code> holds a callable Python
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object, it is invoked with the specified arguments in the manner
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specified <a href="callbacks.html">here</a>. Otherwise, throws <code><a
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href="errors.html#error_already_set-spec">error_already_set</a></code>
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.</dt>
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<dt><b>Returns:</b> An object of unspecified type that holds the Python
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result of the invocation and, when converted to a C++ type
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<code>R</code>, attempts to convert that result object to
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<code>R</code>. If that conversion fails, throws <code><a href=
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"errors.html#error_already_set-spec">error_already_set</a></code>
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.</dt>
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</dl>
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<h3><a name="wrapper-spec"></a>Class template <code>wrapper</code></h3>
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<p>Deriving your wrapper class from both ``T`` <i>and</i>
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``wrapper<T> makes writing that derived class easier.</p>
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<h4><a name="wrapper-spec-synopsis"></a>Class template
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<code>wrapper</code> synopsis</h4>
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<pre>
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namespace boost
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{
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class wrapper
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{
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protected:
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override get_override(char const* name) const;
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};
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};
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</pre>
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<h4><a name="wrapper-spec-observers"></a>Class <code>wrapper</code>
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observer functions</h4>
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<pre>
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override get_override(char const* name) const;
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</pre>
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<dl class="function-semantics">
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<dt><b>Requires:</b> <code>name</code> is a <a href=
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"definitions.html#ntbs">ntbs</a>.</dt>
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<dt><b>Returns:</b> If <code>*this</code> is the C++ base class
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subobject of a Python derived class instance that overrides the named
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function, returns an <code>override</code> object that delegates to the
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Python override. Otherwise, returns an <code>override</code> object
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that holds <code>None</code>.</dt>
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</dl>
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<h2><a name="examples"></a>Example</h2>
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<pre>
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#include <boost/python/module.hpp>
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#include <boost/python/class.hpp>
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#include <boost/python/wrapper.hpp>
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#include <boost/python/call.hpp>
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using namespace boost::python;
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// Class with one pure virtual function
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struct P
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{
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virtual ~P(){}
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virtual char const* f() = 0;
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char const* g() { return "P::g()"; }
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};
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struct PCallback : P, wrapper<P>
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{
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char const* f()
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{
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#if BOOST_WORKAROUND(BOOST_MSVC, <= 1300) // Workaround for vc6/vc7
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return call<char const*>(this->get_override("f").ptr());
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#else
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return this->get_override("f")();
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#endif
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}
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};
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// Class with one non-pure virtual function
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struct A
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{
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virtual ~A(){}
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virtual char const* f() { return "A::f()"; }
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};
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struct ACallback : A, wrapper<A>
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{
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char const* f()
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{
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if (override f = this->get_override("f"))
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#if BOOST_WORKAROUND(BOOST_MSVC, <= 1300) // Workaround for vc6/vc7
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return call<char const*>(f.ptr());
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#else
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return f();
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#endif
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return A::f();
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}
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char const* default_f() { return this->A::f(); }
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};
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BOOST_PYTHON_MODULE_INIT(polymorphism)
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{
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class_<PCallback,boost::noncopyable>("P")
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.def("f", pure_virtual(&P::f))
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;
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class_<ACallback,boost::noncopyable>("A")
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.def("f", &A::f, &ACallback::default_f)
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;
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}
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</pre>
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<p>Revised
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<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
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31 October, 2004
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<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
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<p><i>© Copyright <a href="../../../../people/dave_abrahams.htm">Dave
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Abrahams</a> 2004</i>
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