rnative.png" alt="alternative"></span>
        </p></blockquote></div>
<p>
        PEGs allow for ambiguity in the alternatives. In the expression above, both
        <code class="computeroutput"><span class="identifier">a</span></code> or <code class="computeroutput"><span class="identifier">b</span></code>
        can both match an input string. However, only the first matching alternative
        is valid. As noted, there can only be one valid parse tree.
      </p>
<h5>
<a name="spirit.abstracts.parsing