Short and very sharp

Hou Yifan leads the 2016 Women's World Championship 2.5-1.5 after 4 games. Apart from Muzychul losing badly in the 2nd game, the match has been pretty even, with the other 3 games being drawn.
The 4th game of the match, was a pretty sharp draw, with Muzychuk sacrificing material for a perpetual. When I see short repetitions like this I wonder if (a) they have been played before or (b) part of home preparation for one side. In this case I suspect it is the later, although it was White who chose to vary from previous games (including Fischer - Olaffson 1966), with 13.Nd4.


Hou,Yifan - Muzychuk,Mariya [C83]
2016 WOmens World Championship, 06.03.2016

1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Nxe4 6. d4 b5 7. Bb3 d5 8. dxe5 Be6 9. c3 Be7 10. Bc2 Bg4 11. h3 Bh5 12. g4 Bg6 13. Nd4 Nxd4 14. cxd4 h5 15. f3 Ng3 16. Rf2 hxg4 17. Bxg6 Rxh3 18. Qc2 Bc5 19. Qxc5 Rh1+ 20. Kg2 Rh2+ 21. Kg1 Rh1+ 1/2-1/2