access the upper bound of a tau interval
Major Section:  TAU-SYSTEM

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 well-formed interval.

When the domain of an interval is INTEGERP, there are additional constraints on the other components. See make-tau-interval.