simple formatter factory, the filter factory
          requires the user's type to support reading from a stream. This is so because
          the filter factory would have to parse the argument of the filter relation
          from a string.
        </p>
<p>
          But we won't get away with a simple filter factory, because our <code class="computeroutput"><span class="identifier">point</span></code> class doesn't have a sensible ordering
          semantics and thus we cannot define the complete set of