<h2>Why is this an issue?</h2>
<p>This rule allows the definition of custom rules using XPath expressions.</p>
<p>Issues are created depending on the return value of the XPath expression. If the XPath expression returns:</p>
<ul>
  <li> a single or list of AST nodes, then a line issue with the given message is created for each node </li>
  <li> a boolean, then a file issue with the given message is created only if the boolean is true </li>
  <li> anything else, no issue is created </li>
</ul>
<p>Here is an 