class="computeroutput">Domain</code> is the Proto domain to which this expression extension belongs.
        (See <code class="computeroutput"><a class="link" href="boost/proto/domain.html" title="Struct template domain">proto::domain&lt;&gt;</a></code>.)
        Can be preceeded with "<code class="computeroutput">typename</code>" if the specified domain is a dependent type.
      </p>
<p><code class="computeroutput">BOOST_PROTO_BASIC_EXTENDS()</code> adds to its enclosing struct
        exactly one data me