csmath: 16. ...a6
alternative leads to a pawn down endgame with all the pawns on one side.
16. ...b6
alternative leads to somewhat better game for white.
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17. ...b6?! [17. ...e6!! double pawn sacrifice leads to remarkably complex game: 18. Bxa7 exd5, 19. exd5 Nc4! 20. Bxc4 (accepting the second pawn immediately 20. Rxb7?! Nb2, 21. Qd2 Nxd3, 22.Qxd3 Bc6 leads to return of both pawns), 20.... Rxc4, 21. Rxb7 Qc8 with compensation]
19. Qd3 [19. Bf4!? deserves attention]
21. e5!? [Grigoric conceives positional queen sacrifice move but that completely disturbs the position, 21. Nd4 preparatory move was more in order, of course this is too slow for Gligoric]
25. ...Qxe2?! [25. ...Qf5!?, 26. Rbc1 Bb2, 27. Rc4 Qc8, 28. Nc3 Kg7, 29. Be5! f6, 30. Bg3 Ba3, 31. Ne4 /Unclear my further analysis shows about equal endgame with KQBppppp versus KRRNpppp]
26. ... e5 [26. ...Qc4, 27. Rc1 Qxf4, 22. Rxf8 Bxf8, 23. c8Q Kg7 with uncertain game]
27. ...Qa6? [decisive error, 27. ...e4, 28. fxe4 Qxe4, 29. Rc1 Qd4!, 31. Rxd4 Bxd4, 32. Kf1 Rc8, 33. Bf4 Kf8 with uncertain outcome]