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

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 well-formed interval.

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