This is what I expected the optimization to be, except I'm still not sure that I understand the wording of "that match the columns to the right", I'd expect tha

This is what I expected the optimization to be, except I'm still not sure that I understand the wording of "that match the columns to the right", I'd expect that to be "that match the columns to the left", after all, you're checking the existence of (1,* ), not of (* ,2) or (* ,3).