Applies a tactic to the all the goals in the goal-list for which the
tactic succeeds.
DESCRIPTION
Given a list of goals gl, an application of SELECT_LT tac to gl
will try to apply tac to each goal in gl in turn. If no goal lets
tac succeed, the goal state remains unchanged. Otherwise, the goals
for which tac succeeds will generate (possibly empty) list(s) of
new sub-goals. These new sub-goals are pushed onto the front of the
rest of gl.
FAILURE
The application of SELECT_LT to a tactic never fails. The resulting
list-tactic also never fails.