[Retros] Solutions for attempts at captureless castling switchback

[Noam Elkies]

I just wrote:


> the next message gives the intended solutions.


Here they are:

in 19.0:
  1 a4 g6  2 Ra3 Bg7  3 Rh3 Be5  4 d3 Nf6  5 Qd2 0-0  6 Qh6 Kh8  7 Bf4 Rg8
  8 Qf8 b6  9 Rh6 Ba6  10 h4 Qc8  11 Rh3 Qb7  12 Re3 Qf3  13 Bh2 Qh3
  14 g3 Bb5  15 Bg2 Na6  16 Qb8 Kg7  17 Bb7 Rh8  18 Bc8 Kf8  19 b3 Ke8

in 21.5 (switchback is completed after 19.5):
  1 g3 a5  2 Bg2 Ra6  3 Bd5 Rf6  4 Nf3 d6  5 0-0 Qd7  6 Kh1 Qh3
  7 Rg1 Qf1  8 b3 Bh3  9 Ba3 Rf5  10 Qc1 f6  11 Qb2 Kf7  12 Qd4 Kg6
  13 Nc3 Qb1  14 Bc1 Kh5  15 Rg2 Kg4  16 Kg1 h5  17 Kf1 Rh6  18 Ke1 Rg6
  19 Rg1 gRg5  20 Rh1 g6  21 Ng1 Bh6  22 Bf3#

The second one has two additional switchbacks, one (Bc1-a3-c1) as part
of the thematic mechanism and the other (Ng1-f3-g1) in the mating coda. 

--Noam D. Elkies