ember functions, and
        data members necessary to make a struct a valid Proto expression extension. It does <span class="emphasis"><em>not</em></span>
        add any constructors, virtual functions or access control blocks that would render the containing
        struct non-POD.
      </p>
<p>
        <code class="computeroutput">Expr</code> is the Proto expression that the enclosing struct extends.
        <code class="computeroutput">Derived</code> is the type of the enclosing struct.
        <code 