Access the upper bound of a tau interval
It is the case that
(tau-interval-hi (make-tau-interval dom lo-rel lo hi-rel hi)) = hi
For a well-formed interval, hi is either nil, denoting positive
infinity, or a rational number giving the upper bound of the interval. It
must be the case that the upper bound is weakly above the lower bound of a
When the domain of an interval is INTEGERP, there are additional
constraints on the other components. See make-tau-interval.