s generating infinite languages.</p>
<div class="variablelist"><table border="0"><col align="left"/><tbody><tr><td><span class="term"><i><tt>ctxt</tt></i>:</span></td><td>the expressions context</td></tr><tr><td><span class="term"><i><tt>exp</tt></i>:</span></td><td>the englobing expression</td></tr><tr><td><span class="term"><i><tt>sub</tt></i>:</span></td><td>the subexpression</td></tr><tr><td><span class="term"><i><tt>Returns</tt></i>:</span></td><td>the resulting expression or NULL in case of internal 