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