 1 of [0-9] at line 1 char 1.
      `- Failed to match [0-9] at line 1 char 1.
</pre>
<p>This is what parslet calls an <code>#error_tree</code>. Not only the output of
your parser, but also its grammar is constructed like a tree. When things go
wrong, every branch of the tree has its own reasons for not accepting a given
input. The <code>#parse_failure_cause</code> method returns those reasons.</p>
<p>Our grammar has essentially two branches, <code>SUM</code> and
<code>INTEGER</code>. Can you see why all ru