[#34ECF5D7E50506F83CBB36179139274B1E9D1E09]

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        <h2>Function optional_rule</h2>
        
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        <h3>Synopsis</h3>
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            <code>
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        <pre class="source-code cpp">
            template<class Rule>
constexpr
optional_rule_t<Rule>
optional_rule(Rule const& r);
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