an><span class="keyword">typename</span> <span class="identifier">Rule</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">when</span>
        <span class="special">:</span> <a href="http://www.boost.org/doc/libs/release/doc/html/boost/proto/nary_expr.html" target="_top"><code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span></code></a>
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