HEADGOAL : tactic -> list_tactic
The list-tactic which applies a tactic to the first member of a list of goals.
If tac is a tactic, HEADGOAL tac is a
list-tactic which applies the tactic tac to the
first member of a list of goals.
The application of HEADGOAL to a tactic never fails.
The resulting list-tactic fails the goal list is empty or
or finally if tac fails when applied to the first member of the goal list.
Applying a tactic to the first subgoal.
Where tac1 and tac2 are tactics, tac1 THEN_LT HEADGOAL tac2
applies tac1 to a goal, and then applies tac2 to the first resulting