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<h1>Python Enhancement Proposals</h1>
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<li><a href="https://www.python.org/" title="The Python Programming Language">Python</a> » </li>
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<li><a href="../pep-0000/">PEP Index</a> » </li>
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<h1 class="page-title">PEP 215 – String Interpolation</h1>
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<dl class="rfc2822 field-list simple">
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<dt class="field-odd">Author<span class="colon">:</span></dt>
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<dd class="field-odd">Ka-Ping Yee <ping at zesty.ca></dd>
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<dt class="field-even">Status<span class="colon">:</span></dt>
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<dd class="field-even"><abbr title="Replaced by another succeeding PEP">Superseded</abbr></dd>
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<dt class="field-odd">Type<span class="colon">:</span></dt>
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<dd class="field-odd"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd>
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<dt class="field-even">Created<span class="colon">:</span></dt>
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<dd class="field-even">24-Jul-2000</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">2.1</dd>
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<dt class="field-even">Post-History<span class="colon">:</span></dt>
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<dd class="field-even"><p></p></dd>
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<dt class="field-odd">Superseded-By<span class="colon">:</span></dt>
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<dd class="field-odd"><a class="reference external" href="../pep-0292/">292</a></dd>
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</dl>
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<details><summary>Table of Contents</summary><ul class="simple">
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<li><a class="reference internal" href="#abstract">Abstract</a></li>
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<li><a class="reference internal" href="#copyright">Copyright</a></li>
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<li><a class="reference internal" href="#specification">Specification</a></li>
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<li><a class="reference internal" href="#examples">Examples</a></li>
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<li><a class="reference internal" href="#discussion">Discussion</a></li>
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<li><a class="reference internal" href="#security-issues">Security Issues</a></li>
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<li><a class="reference internal" href="#implementation">Implementation</a></li>
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<li><a class="reference internal" href="#references">References</a></li>
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</ul>
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</details></section>
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<section id="abstract">
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<h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2>
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<p>This document proposes a string interpolation feature for Python
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to allow easier string formatting. The suggested syntax change
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is the introduction of a ‘$’ prefix that triggers the special
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interpretation of the ‘$’ character within a string, in a manner
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reminiscent to the variable interpolation found in Unix shells,
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awk, Perl, or Tcl.</p>
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</section>
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<section id="copyright">
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<h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2>
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<p>This document is in the public domain.</p>
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</section>
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<section id="specification">
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<h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2>
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<p>Strings may be preceded with a ‘$’ prefix that comes before the
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leading single or double quotation mark (or triplet) and before
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any of the other string prefixes (‘r’ or ‘u’). Such a string is
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processed for interpolation after the normal interpretation of
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backslash-escapes in its contents. The processing occurs just
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before the string is pushed onto the value stack, each time the
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string is pushed. In short, Python behaves exactly as if ‘$’
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were a unary operator applied to the string. The operation
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performed is as follows:</p>
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<p>The string is scanned from start to end for the ‘$’ character
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(<code class="docutils literal notranslate"><span class="pre">\x24</span></code> in 8-bit strings or <code class="docutils literal notranslate"><span class="pre">\u0024</span></code> in Unicode strings). If there
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are no ‘$’ characters present, the string is returned unchanged.</p>
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<p>Any ‘$’ found in the string, followed by one of the two kinds of
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expressions described below, is replaced with the value of the
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expression as evaluated in the current namespaces. The value is
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converted with <code class="docutils literal notranslate"><span class="pre">str()</span></code> if the containing string is an 8-bit string,
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or with <code class="docutils literal notranslate"><span class="pre">unicode()</span></code> if it is a Unicode string.</p>
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<ol class="arabic simple">
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<li>A Python identifier optionally followed by any number of
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trailers, where a trailer consists of:
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- a dot and an identifier,
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- an expression enclosed in square brackets, or
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- an argument list enclosed in parentheses
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(This is exactly the pattern expressed in the Python grammar
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by “<code class="docutils literal notranslate"><span class="pre">NAME</span> <span class="pre">trailer*</span></code>”, using the definitions in <code class="docutils literal notranslate"><span class="pre">Grammar/Grammar</span></code>.)</li>
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<li>Any complete Python expression enclosed in curly braces.</li>
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</ol>
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<p>Two dollar-signs (“$$”) are replaced with a single “$”.</p>
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</section>
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<section id="examples">
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<h2><a class="toc-backref" href="#examples" role="doc-backlink">Examples</a></h2>
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<p>Here is an example of an interactive session exhibiting the
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expected behaviour of this feature.</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>>>> a, b = 5, 6
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>>> print $'a = $a, b = $b'
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a = 5, b = 6
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>>> $u'uni${a}ode'
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u'uni5ode'
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>>> print $'\$a'
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5
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>>> print $r'\$a'
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\5
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>>> print $'$$$a.$b'
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$5.6
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>>> print $'a + b = ${a + b}'
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a + b = 11
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>>> import sys
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>>> print $'References to $a: $sys.getrefcount(a)'
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References to 5: 15
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>>> print $"sys = $sys, sys = $sys.modules['sys']"
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sys = <module 'sys' (built-in)>, sys = <module 'sys' (built-in)>
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>>> print $'BDFL = $sys.copyright.split()[4].upper()'
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BDFL = GUIDO
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</pre></div>
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</div>
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</section>
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<section id="discussion">
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<h2><a class="toc-backref" href="#discussion" role="doc-backlink">Discussion</a></h2>
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<p>‘$’ is chosen as the interpolation character within the
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string for the sake of familiarity, since it is already used
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for this purpose in many other languages and contexts.</p>
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<p>It is then natural to choose ‘$’ as a prefix, since it is a
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mnemonic for the interpolation character.</p>
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<p>Trailers are permitted to give this interpolation mechanism
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even more power than the interpolation available in most other
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languages, while the expression to be interpolated remains
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clearly visible and free of curly braces.</p>
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<p>‘$’ works like an operator and could be implemented as an
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operator, but that prevents the compile-time optimization
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and presents security issues. So, it is only allowed as a
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string prefix.</p>
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</section>
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<section id="security-issues">
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<h2><a class="toc-backref" href="#security-issues" role="doc-backlink">Security Issues</a></h2>
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<p>“$” has the power to eval, but only to eval a literal. As
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described here (a string prefix rather than an operator), it
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introduces no new security issues since the expressions to be
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evaluated must be literally present in the code.</p>
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</section>
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<section id="implementation">
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<h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2>
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<p>The <code class="docutils literal notranslate"><span class="pre">Itpl</span></code> module at <a class="footnote-reference brackets" href="#id2" id="id1">[1]</a> provides a
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prototype of this feature. It uses the <code class="docutils literal notranslate"><span class="pre">tokenize</span></code> module to find
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the end of an expression to be interpolated, then calls <code class="docutils literal notranslate"><span class="pre">eval()</span></code>
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on the expression each time a value is needed. In the prototype,
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the expression is parsed and compiled again each time it is
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evaluated.</p>
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<p>As an optimization, interpolated strings could be compiled
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directly into the corresponding bytecode; that is,</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$'a = $a, b = $b'
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</pre></div>
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</div>
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<p>could be compiled as though it were the expression</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="s1">'a = '</span> <span class="o">+</span> <span class="nb">str</span><span class="p">(</span><span class="n">a</span><span class="p">)</span> <span class="o">+</span> <span class="s1">', b = '</span> <span class="o">+</span> <span class="nb">str</span><span class="p">(</span><span class="n">b</span><span class="p">))</span>
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</pre></div>
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</div>
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<p>so that it only needs to be compiled once.</p>
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</section>
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<section id="references">
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<h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2>
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<aside class="footnote-list brackets">
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<aside class="footnote brackets" id="id2" role="doc-footnote">
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<dt class="label" id="id2">[<a href="#id1">1</a>]</dt>
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<dd><a class="reference external" href="http://www.lfw.org/python/Itpl.py">http://www.lfw.org/python/Itpl.py</a></aside>
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</aside>
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</section>
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</section>
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<hr class="docutils" />
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<p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0215.rst">https://github.com/python/peps/blob/main/peps/pep-0215.rst</a></p>
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<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0215.rst">2023-09-09 17:39:29 GMT</a></p>
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<h2>Contents</h2>
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<ul>
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<li><a class="reference internal" href="#abstract">Abstract</a></li>
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<li><a class="reference internal" href="#copyright">Copyright</a></li>
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<li><a class="reference internal" href="#specification">Specification</a></li>
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<li><a class="reference internal" href="#examples">Examples</a></li>
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<li><a class="reference internal" href="#discussion">Discussion</a></li>
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<li><a class="reference internal" href="#security-issues">Security Issues</a></li>
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<li><a class="reference internal" href="#implementation">Implementation</a></li>
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<li><a class="reference internal" href="#references">References</a></li>
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</ul>
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