First move of bK?

Originally posted by BigDoggProblem
The bN returned to b1 to un-promote, so I'll continue fetofs' retraction:

-9.Rb4-b5 b2-b1=N
-10.Rb3-b4 a3xBb2
-11.Bc1-b2 a4-a3
-12.Rb1-b3 a5-a4
-13.b2xBc3!

[fen]8/1p2p3/2p1ppp1/p6b/4P2p/2b1Pk1K/PPPPNPnP/1RBBrRrQ w - - 0 1[/fen]

Puts another Black piece on the board, just in time. (Why couldn't he continue retracting the a-pawn? The answer will be apparent later...)

-13...Be5-c3
-14.Ra1-b1 Bf4-e5
-15.Rb1-a1 Bh6-f4
-16.Ra1-b1 Bf8-h6
-17.Rb1-a1 g7-g6
-18.Ra1-b1 Be8-h5
-19.Rb1-a1 Bd7-e8
-20.Ra1-b1 Bc8-d7
-21.Rb1-a1 d7xNe6
-22.Ne6-f4 h5-h4
-23.Ra1-b1 Nh4-g2
-24.Ng2-f4+ Nf5-h4
-25.Kh4-h3 Nd4-f5
-26.Kg5-h4
Forced - the wK has no other escape route.
-26...f7-f6+



The key position. bK must have got out via d8. White's King can move along the fifth rank. Eventually, black's King can escape through g4. The pawns on a5 and h5 must remain there until Re1 and Rg1 return home. Pc6 must remain until bQ and bK return home.