Further thinking about this, I am now of the opinion that
intval(1e309) = Empty is not safe enough.
Arnold's proposal mentions an accompanying error flag, but also the
sentence
"I expect that the flag will never be inspected", with which I agree.
I would definitely prefer it to return Entire.
At least this way, the basic containment property is at the safest level,
while if you use Empty, an error could easily go undetected:
Take the example of an algorithm computing the hull of a bunch of
floating-point
values. The algorithm iterates over the values, converts them to interval,
and accumulates using hull(). If a floating-point value is 1e309 or +Inf,
and it gets converted to Empty, then it has gone unnoticed.
If you use Entire, you will get useless computations, which will force
you to
look at the problem, but it will not go unnoticed.