 <p>This example does not deal with a full case, but with a situation that
  can occur often. Sometimes, it can be useful to change the policies of an
  interval by converting it to another type. For example, this happens when
  you use an unprotected version of the interval type in order to speed up
  the computations; it is a change of the rounding policy. It also happens
  when you want to temporarily allow empty intervals to be created; it is a
  change of the checking policy. These changes should not b