Dominosa- container classes C++
<p>I'm doing an exercise- Dominosa- where I have to use recursive backtracking to solve Dominosa boards. On a board of random numbers, the numbers need to be paired, no pairing can be repeated. </p>

<p>The problem is that the class header files don't seem to allow the use of the container classes- or at least not in the way that I'm trying to. So far I've tried sets within sets, maps within sets, vectors within sets etc- to keep track of pairs of numbers that have been selected. But I always get the error "Invalid operands to binary expression", which links to this;</p>

<pre><code> /// One of the @link s20_3_3_comparisons comparison functors@endlink.
      template &lt;class _Tp&gt;
       struct less : public binary_function&lt;_Tp, _Tp, bool&gt;
        {
          bool
          operator()(const _Tp&amp; __x, const _Tp&amp; __y) const
          { return __x &lt; __y; }
       };
</code></pre>

<p>Which is in stl_function.h. So this;</p>

<pre><code>Map&lt;coord, int&gt; numbers;
</code></pre>

<p>doesnt create an error, whilst this;</p>

<pre><code>Set&lt;Set&lt;Map&lt;coord, int&gt; &gt; &gt; pairings;
</code></pre>

<p>Creates an error. </p>

<p>Currently I don't see how to solve this without keeping track of pairings...?</p>
