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<title>Rule Parsers</title>
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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_parser.tutorial.rule_parsers"></a><a class="link" href="rule_parsers.html" title="Rule Parsers">Rule Parsers</a>
</h3></div></div></div>
<p>
This example is very similar to the others we've seen so far. This one is
different only because it uses a <code class="computeroutput"><a class="link" href="../../boost/parser/rule.html" title="Struct template rule">rule</a></code>. As an analogy, think
of a parser like <code class="computeroutput"><a class="link" href="../../boost/parser/char_.html" title="Global char_">char_</a></code>
or <code class="computeroutput"><a class="link" href="../../boost/parser/double_.html" title="Global double_">double_</a></code>
as an individual line of code, and a <code class="computeroutput"><a class="link" href="../../boost/parser/rule.html" title="Struct template rule">rule</a></code> as a function. Like a
function, a <code class="computeroutput"><a class="link" href="../../boost/parser/rule.html" title="Struct template rule">rule</a></code>
has its own name, and can even be forward declared. Here is how we define
a <code class="computeroutput"><a class="link" href="../../boost/parser/rule.html" title="Struct template rule">rule</a></code>,
which is analogous to forward declaring a function:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">bp</span><span class="special">::</span><span class="identifier">rule</span><span class="special">&lt;</span><span class="keyword">struct</span> <span class="identifier">doubles</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;&gt;</span> <span class="identifier">doubles</span> <span class="special">=</span> <span class="string">"doubles"</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
This declares the rule itself. The <code class="computeroutput"><a class="link" href="../../boost/parser/rule.html" title="Struct template rule">rule</a></code> is a parser, and we can
immediately use it in other parsers. That definition is pretty dense; take
note of these things:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
The first template parameter is a tag type <code class="computeroutput"><span class="keyword">struct</span>
<span class="identifier">doubles</span></code>. Here we've declared
the tag type and used it all in one go; you can also use a previously
declared tag type.
</li>
<li class="listitem">
The second template parameter is the attribute type of the parser. If
you don't provide this, the rule will have no attribute.
</li>
<li class="listitem">
This rule object itself is called <code class="computeroutput"><span class="identifier">doubles</span></code>.
</li>
<li class="listitem">
We've given <code class="computeroutput"><span class="identifier">doubles</span></code> the
diagnstic text <code class="computeroutput"><span class="string">"doubles"</span></code>
so that Boost.Parser knows how to refer to it when producing a trace
of the parser during debugging.
</li>
</ul></div>
<p>
Ok, so if <code class="computeroutput"><span class="identifier">doubles</span></code> is a parser,
what does it do? We define the rule's behavior by defining a separate parser
that by now should look pretty familiar:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">auto</span> <span class="keyword">const</span> <span class="identifier">doubles_def</span> <span class="special">=</span> <span class="identifier">bp</span><span class="special">::</span><span class="identifier">double_</span> <span class="special">%</span> <span class="char">','</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
This is analogous to writing a definition for a forward-declared function.
Note that we used the name <code class="computeroutput"><span class="identifier">doubles_def</span></code>.
Right now, the <code class="computeroutput"><span class="identifier">doubles</span></code> rule
parser and the <code class="computeroutput"><span class="identifier">doubles_def</span></code>
non-rule parser have no connection to each other. That's intentional —
we want to be able to define them separately. To connect them, we declare
functions with an interface that Boost.Parser understands, and use the tag
type <code class="computeroutput"><span class="keyword">struct</span> <span class="identifier">doubles</span></code>
to connect them together. We use a macro for that:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">BOOST_PARSER_DEFINE_RULES</span><span class="special">(</span><span class="identifier">doubles</span><span class="special">);</span>
</pre>
<p>
</p>
<p>
This macro expands to the code necessary to make the rule <code class="computeroutput"><span class="identifier">doubles</span></code>
and its parser <code class="computeroutput"><span class="identifier">doubles_def</span></code>
work together. The <code class="computeroutput"><span class="identifier">_def</span></code> suffix
is a naming convention that this macro relies on to work. The tag type allows
the rule parser, <code class="computeroutput"><span class="identifier">doubles</span></code>,
to call one of these overloads when used as a parser.
</p>
<p>
<code class="computeroutput"><a class="link" href="../../BOOST_PARSER_DEFINE_RULES.html" title="Macro BOOST_PARSER_DEFINE_RULES">BOOST_PARSER_DEFINE_RULES</a></code>
expands to two overloads of a function called <code class="computeroutput"><span class="identifier">parse_rule</span><span class="special">()</span></code>. In the case above, the overloads each
take a <code class="computeroutput"><span class="keyword">struct</span> <span class="identifier">doubles</span></code>
parameter (to distinguish them from the other overloads of <code class="computeroutput"><span class="identifier">parse_rule</span><span class="special">()</span></code>
for other rules) and parse using <code class="computeroutput"><span class="identifier">doubles_def</span></code>.
You will never need to call any overload of <code class="computeroutput"><span class="identifier">parse_rule</span><span class="special">()</span></code> yourself; it is used internally by the
parser that implements <code class="computeroutput"><a class="link" href="../../boost/parser/rule.html" title="Struct template rule">rules</a></code>, <code class="computeroutput"><span class="identifier">rule_parser</span></code>.
</p>
<p>
Here is the definition of the macro that is expanded for each rule:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_PARSER_DEFINE_IMPL</span><span class="special">(</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">rule_name_</span><span class="special">)</span> <span class="special">\</span>
<span class="keyword">template</span><span class="special">&lt;</span> <span class="special">\</span>
<span class="keyword">typename</span> <span class="identifier">Iter</span><span class="special">,</span> <span class="special">\</span>
<span class="keyword">typename</span> <span class="identifier">Sentinel</span><span class="special">,</span> <span class="special">\</span>
<span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">,</span> <span class="special">\</span>
<span class="keyword">typename</span> <span class="identifier">SkipParser</span><span class="special">&gt;</span> <span class="special">\</span>
<span class="keyword">decltype</span><span class="special">(</span><span class="identifier">rule_name_</span><span class="special">)::</span><span class="identifier">parser_type</span><span class="special">::</span><span class="identifier">attr_type</span> <span class="identifier">parse_rule</span><span class="special">(</span> <span class="special">\</span>
<span class="keyword">decltype</span><span class="special">(</span><span class="identifier">rule_name_</span><span class="special">)::</span><span class="identifier">parser_type</span><span class="special">::</span><span class="identifier">tag_type</span> <span class="special">*,</span> <span class="special">\</span>
<span class="identifier">Iter</span> <span class="special">&amp;</span> <span class="identifier">first</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">Sentinel</span> <span class="identifier">last</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">Context</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">context</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">SkipParser</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">skip</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">parser</span><span class="special">::</span><span class="identifier">detail</span><span class="special">::</span><span class="identifier">flags</span> <span class="identifier">flags</span><span class="special">,</span> <span class="special">\</span>
<span class="keyword">bool</span> <span class="special">&amp;</span> <span class="identifier">success</span><span class="special">,</span> <span class="special">\</span>
<span class="keyword">bool</span> <span class="special">&amp;</span> <span class="identifier">dont_assign</span><span class="special">)</span> <span class="special">\</span>
<span class="special">{</span> <span class="special">\</span>
<span class="keyword">auto</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">parser</span> <span class="special">=</span> <span class="identifier">BOOST_PARSER_PP_CAT</span><span class="special">(</span><span class="identifier">rule_name_</span><span class="special">,</span> <span class="identifier">_def</span><span class="special">);</span> <span class="special">\</span>
<span class="keyword">using</span> <span class="identifier">attr_t</span> <span class="special">=</span> <span class="special">\</span>
<span class="keyword">decltype</span><span class="special">(</span><span class="identifier">parser</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">,</span> <span class="identifier">context</span><span class="special">,</span> <span class="identifier">skip</span><span class="special">,</span> <span class="identifier">flags</span><span class="special">,</span> <span class="identifier">success</span><span class="special">));</span> <span class="special">\</span>
<span class="keyword">using</span> <span class="identifier">attr_type</span> <span class="special">=</span> <span class="keyword">decltype</span><span class="special">(</span><span class="identifier">rule_name_</span><span class="special">)::</span><span class="identifier">parser_type</span><span class="special">::</span><span class="identifier">attr_type</span><span class="special">;</span> <span class="special">\</span>
<span class="keyword">if</span> <span class="keyword">constexpr</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">parser</span><span class="special">::</span><span class="identifier">detail</span><span class="special">::</span><span class="identifier">is_nope_v</span><span class="special">&lt;</span><span class="identifier">attr_t</span><span class="special">&gt;)</span> <span class="special">{</span> <span class="special">\</span>
<span class="identifier">dont_assign</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> <span class="special">\</span>
<span class="identifier">parser</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">,</span> <span class="identifier">context</span><span class="special">,</span> <span class="identifier">skip</span><span class="special">,</span> <span class="identifier">flags</span><span class="special">,</span> <span class="identifier">success</span><span class="special">);</span> <span class="special">\</span>
<span class="keyword">return</span> <span class="special">{};</span> <span class="special">\</span>
<span class="special">}</span> <span class="keyword">else</span> <span class="keyword">if</span> <span class="keyword">constexpr</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">is_same_v</span><span class="special">&lt;</span><span class="identifier">attr_type</span><span class="special">,</span> <span class="identifier">attr_t</span><span class="special">&gt;)</span> <span class="special">{</span> <span class="special">\</span>
<span class="keyword">return</span> <span class="identifier">parser</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">,</span> <span class="identifier">context</span><span class="special">,</span> <span class="identifier">skip</span><span class="special">,</span> <span class="identifier">flags</span><span class="special">,</span> <span class="identifier">success</span><span class="special">);</span> <span class="special">\</span>
<span class="special">}</span> <span class="keyword">else</span> <span class="keyword">if</span> <span class="keyword">constexpr</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">is_constructible_v</span><span class="special">&lt;</span><span class="identifier">attr_type</span><span class="special">,</span> <span class="identifier">attr_t</span><span class="special">&gt;)</span> <span class="special">{</span> <span class="special">\</span>
<span class="keyword">return</span> <span class="identifier">attr_type</span><span class="special">(</span> <span class="special">\</span>
<span class="identifier">parser</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">,</span> <span class="identifier">context</span><span class="special">,</span> <span class="identifier">skip</span><span class="special">,</span> <span class="identifier">flags</span><span class="special">,</span> <span class="identifier">success</span><span class="special">));</span> <span class="special">\</span>
<span class="special">}</span> <span class="keyword">else</span> <span class="special">{</span> <span class="special">\</span>
<span class="identifier">attr_type</span> <span class="identifier">attr</span><span class="special">{};</span> <span class="special">\</span>
<span class="identifier">parser</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">,</span> <span class="identifier">context</span><span class="special">,</span> <span class="identifier">skip</span><span class="special">,</span> <span class="identifier">flags</span><span class="special">,</span> <span class="identifier">success</span><span class="special">,</span> <span class="identifier">attr</span><span class="special">);</span> <span class="special">\</span>
<span class="keyword">return</span> <span class="identifier">attr</span><span class="special">;</span> <span class="special">\</span>
<span class="special">}</span> <span class="special">\</span>
<span class="special">}</span> <span class="special">\</span>
<span class="special">\</span>
<span class="keyword">template</span><span class="special">&lt;</span> <span class="special">\</span>
<span class="keyword">typename</span> <span class="identifier">Iter</span><span class="special">,</span> <span class="special">\</span>
<span class="keyword">typename</span> <span class="identifier">Sentinel</span><span class="special">,</span> <span class="special">\</span>
<span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">,</span> <span class="special">\</span>
<span class="keyword">typename</span> <span class="identifier">SkipParser</span><span class="special">,</span> <span class="special">\</span>
<span class="keyword">typename</span> <span class="identifier">Attribute</span><span class="special">&gt;</span> <span class="special">\</span>
<span class="keyword">void</span> <span class="identifier">parse_rule</span><span class="special">(</span> <span class="special">\</span>
<span class="keyword">decltype</span><span class="special">(</span><span class="identifier">rule_name_</span><span class="special">)::</span><span class="identifier">parser_type</span><span class="special">::</span><span class="identifier">tag_type</span> <span class="special">*,</span> <span class="special">\</span>
<span class="identifier">Iter</span> <span class="special">&amp;</span> <span class="identifier">first</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">Sentinel</span> <span class="identifier">last</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">Context</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">context</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">SkipParser</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">skip</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">parser</span><span class="special">::</span><span class="identifier">detail</span><span class="special">::</span><span class="identifier">flags</span> <span class="identifier">flags</span><span class="special">,</span> <span class="special">\</span>
<span class="keyword">bool</span> <span class="special">&amp;</span> <span class="identifier">success</span><span class="special">,</span> <span class="special">\</span>
<span class="keyword">bool</span> <span class="special">&amp;</span> <span class="identifier">dont_assign</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">Attribute</span> <span class="special">&amp;</span> <span class="identifier">retval</span><span class="special">)</span> <span class="special">\</span>
<span class="special">{</span> <span class="special">\</span>
<span class="keyword">auto</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">parser</span> <span class="special">=</span> <span class="identifier">BOOST_PARSER_PP_CAT</span><span class="special">(</span><span class="identifier">rule_name_</span><span class="special">,</span> <span class="identifier">_def</span><span class="special">);</span> <span class="special">\</span>
<span class="keyword">using</span> <span class="identifier">attr_t</span> <span class="special">=</span> <span class="special">\</span>
<span class="keyword">decltype</span><span class="special">(</span><span class="identifier">parser</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">,</span> <span class="identifier">context</span><span class="special">,</span> <span class="identifier">skip</span><span class="special">,</span> <span class="identifier">flags</span><span class="special">,</span> <span class="identifier">success</span><span class="special">));</span> <span class="special">\</span>
<span class="keyword">if</span> <span class="keyword">constexpr</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">parser</span><span class="special">::</span><span class="identifier">detail</span><span class="special">::</span><span class="identifier">is_nope_v</span><span class="special">&lt;</span><span class="identifier">attr_t</span><span class="special">&gt;)</span> <span class="special">{</span> <span class="special">\</span>
<span class="identifier">parser</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">,</span> <span class="identifier">context</span><span class="special">,</span> <span class="identifier">skip</span><span class="special">,</span> <span class="identifier">flags</span><span class="special">,</span> <span class="identifier">success</span><span class="special">);</span> <span class="special">\</span>
<span class="special">}</span> <span class="keyword">else</span> <span class="special">{</span> <span class="special">\</span>
<span class="identifier">parser</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">,</span> <span class="identifier">context</span><span class="special">,</span> <span class="identifier">skip</span><span class="special">,</span> <span class="identifier">flags</span><span class="special">,</span> <span class="identifier">success</span><span class="special">,</span> <span class="identifier">retval</span><span class="special">);</span> <span class="special">\</span>
<span class="special">}</span> <span class="special">\</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
Now that we have the <code class="computeroutput"><span class="identifier">doubles</span></code>
parser, we can use it like we might any other parser:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">auto</span> <span class="keyword">const</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">bp</span><span class="special">::</span><span class="identifier">parse</span><span class="special">(</span><span class="identifier">input</span><span class="special">,</span> <span class="identifier">doubles</span><span class="special">,</span> <span class="identifier">bp</span><span class="special">::</span><span class="identifier">ws</span><span class="special">);</span>
</pre>
<p>
</p>
<p>
The full program:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">parser</span><span class="special">/</span><span class="identifier">parser</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">deque</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span>
<span class="keyword">namespace</span> <span class="identifier">bp</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">parser</span><span class="special">;</span>
<span class="identifier">bp</span><span class="special">::</span><span class="identifier">rule</span><span class="special">&lt;</span><span class="keyword">struct</span> <span class="identifier">doubles</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;&gt;</span> <span class="identifier">doubles</span> <span class="special">=</span> <span class="string">"doubles"</span><span class="special">;</span>
<span class="keyword">auto</span> <span class="keyword">const</span> <span class="identifier">doubles_def</span> <span class="special">=</span> <span class="identifier">bp</span><span class="special">::</span><span class="identifier">double_</span> <span class="special">%</span> <span class="char">','</span><span class="special">;</span>
<span class="identifier">BOOST_PARSER_DEFINE_RULES</span><span class="special">(</span><span class="identifier">doubles</span><span class="special">);</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Please enter a list of doubles, separated by commas. "</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">input</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">getline</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cin</span><span class="special">,</span> <span class="identifier">input</span><span class="special">);</span>
<span class="keyword">auto</span> <span class="keyword">const</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">bp</span><span class="special">::</span><span class="identifier">parse</span><span class="special">(</span><span class="identifier">input</span><span class="special">,</span> <span class="identifier">doubles</span><span class="special">,</span> <span class="identifier">bp</span><span class="special">::</span><span class="identifier">ws</span><span class="special">);</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">result</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"You entered:\n"</span><span class="special">;</span>
<span class="keyword">for</span> <span class="special">(</span><span class="keyword">double</span> <span class="identifier">x</span> <span class="special">:</span> <span class="special">*</span><span class="identifier">result</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">x</span> <span class="special">&lt;&lt;</span> <span class="string">"\n"</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span> <span class="keyword">else</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Parse failure.\n"</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
All this is intended to introduce the notion of <code class="computeroutput"><a class="link" href="../../boost/parser/rule.html" title="Struct template rule">rules</a></code>. It still may be a bit
unclear why you would want to use <code class="computeroutput"><a class="link" href="../../boost/parser/rule.html" title="Struct template rule">rules</a></code>. The use cases for, and
lots of detail about, <code class="computeroutput"><a class="link" href="../../boost/parser/rule.html" title="Struct template rule">rules</a></code> is in a later section,
<a class="link" href="more_about_rules.html" title="More About Rules">More About Rules</a>.
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
The existence of <code class="computeroutput"><a class="link" href="../../boost/parser/rule.html" title="Struct template rule">rules</a></code>
means that will probably never have to write a low-level parser. You can
just put existing parsers together into <code class="computeroutput"><a class="link" href="../../boost/parser/rule.html" title="Struct template rule">rules</a></code> instead.
</p></td></tr>
</table></div>
</div>
<div class="copyright-footer">Copyright © 2020 T. Zachary Laine<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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>)
</p>
</div>
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