Applies a tactic to all subgoals produced by a tactic or list-tactic.
If T1 and T2 are tactics, T1 THEN T2 is a tactic which applies T1 to a
goal, then applies the tactic T2 to all the subgoals generated. If T1
solves the goal then T2 is never applied.
Alternatively, T1 may be a list-tactic which is applied to an initial list of
The application of THEN to a pair of tactics never fails.
The resulting tactic fails if T1 fails when applied to the goal, or if
T2 does when applied to any of the resulting subgoals.
Although normally used to sequence tactics which generate a single subgoal,
it is worth remembering that it is sometimes useful to apply the same tactic
to multiple subgoals; sequences like the following: