Nimzo-Indian Defense: Keres Variation

1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 4.e3 d5 5.a3 Bxc3+ 6.bxc3 c5 7.cxd5 exd5 8.Bd3 O-O 9.Ne2 b6