[Retros] The same or not the same: that is the question

[Michael Niermann-Rossi]

Let me add a new aspect:

Assume we have an endgame K+N+B vs. K, White just has taken the last black
piece. The black King escapes to the safe corner in 10 moves. White starts
the well known technique to force him into the unsafe corner, but makes a
mistake. Black again escapes to the safe corner. Then the same happens again
and we get the "same" position as after move 10, i.e. all pieces are on the
same squares. But now we are at move 49.

In the position after move 10 White can (and should) win. In the position
after move 49 Black can easily force a draw. would you call them "the same"?

Maybe the following can solve all the problems:

I think there is a difference between the "position" in the chess rules and
the "position" in the common chess language. Maybe this is my personal
problem, since I'm not a native English speaker, but I don't think so. 

The chess rules use "position" in the meaning of "board configuration",
consisting of the position of the pieces, the side to move, the rights to
castle and the e.p. right. The rights just depending on whether a K or R has
moved resp. the last move was a pawn advance of 2 squares. 

We use "position" usually as "snap-shot of a chess game", i.e. the number of
moves since the last capture or pawn advance is important, all positions
since then are important (repetition rule), we think about legality of a
position, other retro aspects etc. So we see it as a node in the tree of all
legal positions (maybe not tree but directed graph, since there are circles)
or as a node of a game, which is a way of this large graph.

In the "board configuration" it is not necessary to know the possible moves
of the K and R, since the rights are represented by boolean variables, which
are true in the initial position and each of them can be set to false once in
a chess game, as Yefim explained (BTW, why don't you use the FEN notation??
Your 1111 is KQkq, your 1001 is Qk. I think FEN is standard, so why to set up
a new notation? And the number of subsets of an n-set is 2^n, too :), so you
get 16 again.).

In the Laws of Chess "position" is almost always used in the meaning of
"board configuration" in a correct way, but only in 9.2 it is used unclear:


> Positions as in (a) and (b) are considered the same, if the same player 

> has the move, pieces of the same kind and colour occupy the same squares,

> and the possible moves of all the pieces of both players are the same.

>

> Positions are not the same if a pawn that could have been captured en

> passant can no longer be captured or if the right to castle has been

> changed temporarily or permanently. 


First, I agree that "temporarily" makes no sense here.

Second, "the possible moves of all the pieces of both players" with no
further explanation is a bad formulation. 
Problems:
1) If White has the move, there is no possible move of a black piece in this 
 
   position.
2) E.g. in the initial position no castling is a possible move.
3) If we include possible moves in the future we run into the problems of the
 
   several examples, where the right to castle exists, but we can prove that 
 
   there is no legal series of moves where the castling is executed.
I suppose they meant "the principle right to castle and the temporary right
to take e.p. at a certain square" Probably they should replace in the second
sentence "position" by "possible moves", remove "temporarily" and add at the
end "and the same otherwise". So I suggest:

The possible moves are not the same if a pawn that could have been captured 
en passant can no longer be captured or if the right to castle has been
changed permanently, and the same otherwise. 

Regards,  Michael



	
		
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