ared to record.
If an attempt is made
to <tt class="docutils literal"><span class="pre">put</span></tt> a key-value pair to a nonexistent <tt class="docutils literal"><span class="pre">dynamic_properties</span></tt> key,
then this function is called with the <tt class="docutils literal"><span class="pre">dynamic_properties</span></tt> key and the
intended property key and value .  If <tt class="docutils literal"><span class="pre">dynamic_properties</span></tt> is
default-constructed, such a <tt class="docut