 minimum (an addition of two intervals will only be two
  additions of numbers).</p>

  <h3>Computing ranges</h3>

  <p>The inputs of your program may be empty intervals or invalid values (for
  example, a database can allow undefined values in some field) and the core
  of your program could also do some non-arithmetic computations that do not
  always propagate empty intervals. For example, in the library, the
  <code>hull</code> function can happily receive an empty interval but not
  generate an empty i