ere</a>.
        </p>
<p>
          The library allows not only adding support for new types, but also associating
          new relations with them. For instance, we can create a new relation "is_in_rectangle"
          that will yield positive if the coordinates fit into a rectangle denoted
          with two points. The filter might look like this:
        </p>
<pre class="programlisting">%Coordinates% is_in_rectangle "{(10, 10) - (20, 20)}"</pre>
<p>
          First, let's define our rectangle class:
  