ttributes of the rule. You've seen that our
          parsers can have an attribute. Recall that the <code class="computeroutput"><span class="identifier">double_</span></code>
          parser has an attribute of <code class="computeroutput"><span class="keyword">double</span></code>.
          To be precise, these are <span class="emphasis"><em>synthesized</em></span> attributes. The
          parser "synthesizes" the attribute value. Think of them as function
          return values.
        </p>
<p>
   