an><span class="char">'+'</span> <span class="special">&gt;&gt;</span> <span class="identifier">term</span><span class="special">)</span> <span class="special">|</span> <span class="special">(</span><span class="char">'-'</span> <span class="special">&gt;&gt;</span> <span class="identifier">term</span><span class="special">));</span>
</pre>
<p>
      Through the magic of expression templates, this is perfectly valid and executable
      C++ code. The production rule <code class="computeroutput"><span class=