[Retros] More than 100 positions leading to the foolish mate. Yefim 10/09/2004

[Mario Richter]

Hello,

for me, the ep and castling fields of the FEN are a 
technical means to tell a chess engine some details about
the history of the game, that led to the piece 
configuration defined by the "piece placement field" and
that could have influence on the future possibilities in
the game.

So the ep field simply says that the last move of the side
not having the move was a double-step by a pawn on the
specified file, and the castlings field says which castling 
rights have been lost during the game (by listing all those
castling rights which have *not* been lost).

Viewed this way (and making use of the fact that most chess
tools allow to enter an arbitrary number in the move number
field, the following strings give you quite different
information about the position they describe:

rnb1kbnr/pppp1ppp/4p3/8/6Pq/5P2/PPPPP2P/RNBQKBNR w KQkq - 0 1
vs.
rnb1kbnr/pppp1ppp/4p3/8/6Pq/5P2/PPPPP2P/RNBQKBNR w Qk - 0 1


andrew buchanan wrote:


> ... there is a paradoxical situation with draw by

> repetition. The implication is that no draw can be claimed

> if some castling right remains.



> If castling is possible in the repeated position, then by 

> your argument when the game ends the third time round, 

> castling rights disappear. So the third position is *not* 

> a repeat of the other two, so in fact the game has *not* 

> ended. So castling is legal, so a draw *is* claimable

> again...


I don't see any paradox here. After the third repetion, the
game is over, which gives us a *new* position with the
property "game has ended".
In this *new* position the castling rights might disappear
or not - but that doesn't change the outcome of the game.


How about white's argumentation in the following example:

Assiac
Adventure in Chess
1951

fen= 2b5/Bk1p4/pP6/2P5/7p/6bK/8/5B2 w - - 0 1

    a   b   c   d   e   f   g   h
  +---+---+---+---+---+---+---+---+
8 |   | . |*b | . |   | . |   | . | 8
  +---+---+---+---+---+---+---+---+
7 | B |*k | . |*p | . |   | . |   | 7
  +---+---+---+---+---+---+---+---+
6 |*p | P |   | . |   | . |   | . | 6
  +---+---+---+---+---+---+---+---+
5 | . |   | P |   | . |   | . |   | 5
  +---+---+---+---+---+---+---+---+
4 |   | . |   | . |   | . |   |*p | 4
  +---+---+---+---+---+---+---+---+
3 | . |   | . |   | . |   |*b | K | 3
  +---+---+---+---+---+---+---+---+
2 |   | . |   | . |   | . |   | . | 2
  +---+---+---+---+---+---+---+---+
1 | . |   | . |   | . | B | . |   | 1  (5+6)
  +---+---+---+---+---+---+---+---+
    a   b   c   d   e   f   g   h

(I do not have the original source, so the following
 dialogue is from memory.)

Playing 1.Bf1-g2+, white claims: Checkmate!

Black replies: No, I will play Pd7-d5 and you will be
               checkmated.

White: But you aren't allowed to make this particular move!

Black: Why not?

White: Because my pawn c5 attacks the square d6, and
       according to the rules, your pawn move can be 
       treated as if it only led to d6.
       But that would leave your king in check, and
       according to another rule, no move is allowed,
       which leaves the king of the moving side in check.

:-)


greetings

   mario