<a href="http://www.boost.org/doc/libs/release/doc/html/boost/proto/when.html" target="_top"><code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span></code></a><span class="special">&lt;</span><a href="http://www.boost.org/doc/libs/release/doc/html/boost/proto/_.html" target="_top"><code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code></a><span class