Access the domain of a tau interval
It is the case that
(tau-interval-dom (make-tau-interval dom lo-rel lo hi-rel hi)) = dom
For a well-formed interval, dom is one of the symbols INTEGERP,
RATIONALP, ACL2-NUMBERP, or NIL. When the domain is NIL
there is no domain restriction.
When the domain is INTEGERP, there are additional constraints on the
other components. See make-tau-interval.