In Gokyo Shumyo, Section 1, Problem 75 / Solution, the following response is untreated:
I believe that the / exchange is unnecessary, and that at already forces . Otherwise at seems to live or get out very quickly. So here, IMHO, is the main line.
The variations at are similar to Gokyo Shumyo, Section 1, Problem 75 / Solution.
Other responses at run into shortage of liberties.