uires the two type parameters to model
property map concepts, but also makes some extra assumptions.
<tt class="docutils literal"><span class="pre">AgeMap</span></tt> and <tt class="docutils literal"><span class="pre">GPAMap</span></tt> must have the same key type, and that type must be
constructable from a string.  Furthermore, <tt class="docutils literal"><span class="pre">AgeMap</span></tt>'s value type must be
constructable from an <tt class="docutils literal"><span class="pre">int</span></tt>.  Althoug