es. If <tt class="docutils literal"><span class="pre">Value</span></tt> is <tt class="docutils literal"><span class="pre">std::string</span></tt>, then the
property map's value type must either be <tt class="docutils literal"><span class="pre">std::string</span></tt> or model
OutputStreamable.  In the latter case, the <tt class="docutils literal"><span class="pre">get</span></tt> function converts the
value to a string.  If no matching property map is found,
<tt class="docutils literal"><span class="pre">dy