de class="computeroutput"><span class="identifier">double_</span></code> used twice, once for each number.
          If you are used to see the <code class="computeroutput"><span class="char">'&gt;&gt;'</span></code>
          operator for concatenating two parsers in <span class="emphasis"><em>Spirit.Qi</em></span>
          you might wonder, what's that <code class="computeroutput"><span class="char">'&lt;&lt;'</span></code>
          operator doing in there? We decided to distinguish generating and parsi