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After White's 13 move. If Black plays pawn takes pawn at a4 White likely planned to
recapture and start piling the pressure on Black's isolated a pawn. Note that the White
Queen is already on the key diagonal. White's b and c pawns will provide more attacking
leverage than Black can counter. Black sees all this and counters with the electrifying  
push of the pawn to b4. Now the White Queenside Knight must either counterattack from
the marginal square of a2 (a Knight in the center of the board can attack 8 squares - a
Knight at a2 can attack only 3) or initiate an exchange of pieces at d5.
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