Instead I approached at , but Black's extension makes a nice moyo, which is difficult to invade. I thought is bad and should be at a, to prevent White's development on the fourth line and I still think so, but I didn't find support by gobase analysis, where both moves are about as frequent.
Let's play a few moves on the right. The crux of this BQM is . I thought it was better than a, because that move was pincered in the game (with different conditions on the right side).
I think is urgent. Our analysis of Black's attack at a proved it to be devastating and moreover, White b would be a strong follow-up of if elsewhere. After White seems to have a good opening but I can't see a definite advantage.