[#CC140B4C18D2616EFD643FC4A88449C611695CC4]

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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 R_>
constexpr
optional_rule_t<R_>
optional_rule(R_ const& r);
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