nterval if the other input is valid. The
  <code>intersect</code> function is also able to produce empty intervals if
  the intervals do not overlap. In that case, it is not really interesting if
  an exception is thrown each time an empty interval is produced or an
  invalid value is used; it would be better to generate and propagate empty
  intervals. So you need to change the checking policy to something like
  <code>interval_lib::checking_base&lt;T&gt;</code>.</p>

  <h3>Switching interval types</h3>

 