change to another goal.
(change-goal (main . 1)) -- change to the goal (main . 1)
change-goal -- change to the next-to-top goal
(change-goal &optional goal-name end-flg)
Change to the goal with the name goal-name, i.e. make it the current
goal. However, if goal-name is nil or is not supplied, then it
defaults to the next-to-top goal, i.e., the second goal in the stack of goals.
If end-flg is supplied and not nil, then move the current goal to
the end of the goal stack; else merely swap it with the next-to-top goal.
Also see ACL2-pc::cg.