THEN_TCLThm_cont.THEN_TCL : (thm_tactical * thm_tactical -> thm_tactical)
Composes two theorem-tacticals.
If ttl1 and ttl2 are two theorem-tacticals,
ttl1 THEN_TCL ttl2 is a theorem-tactical which composes
their effect; that is, if:
ttl1 ttac th1 = ttac th2
and
ttl2 ttac th2 = ttac th3
then
(ttl1 THEN_TCL ttl2) ttac th1 = ttac th3
The application of THEN_TCL to a pair of
theorem-tacticals never fails.