Access the upper bound relation of a tau interval
It is the case that
(tau-interval-hi-rel (make-tau-interval dom lo-rel lo hi-rel hi)) = hi-rel
For a well-formed interval, hi-rel is a Boolean, where t denotes
the < (strong inequality or ``less-than'') relation and nil
denotes <= (weak inequality or ``less-than-or-equal'') relation
between the elements of the interval and the upper bound.
When the domain of an interval is INTEGERP, there are additional
constraints on the other components. See make-tau-interval.