output"><span class="identifier">init</span></code>,
                  permitting lazy construction of the message.
                </p>
                <p>
                  The constructor may optionally require that <code class="computeroutput"><span class="identifier">h</span></code>
                  and <code class="computeroutput"><span class="identifier">v</span></code> are <code class="computeroutput"><span class="keyword">const</span></code> references, with these consequences:
                </p