A question on the sicilian najdorf.
Recetly i have been facing the sicilian najdorf in the tornaments and the moves always go like this...1.e4 c5 2. nf3 d6 3. d4 cxd4 4. nxd4 nf6
5.nc3 e6 6.Bc4 a6.
i was wondering why after move 6, a4 is not given to prevent b5.
it seems that a4 is a good move. what is your opinion on this ?? thanks for the help in advance.