Julia's Fairies

No.568 (PR)

Paul Rãican


Original Problems, Julia’s Fairies – 2014 (II): May – August

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No.568 by Paul Rãican – A funny problem with two different solutions by two different programs! 

Author’s guess is “I believe that Whites have Sb1-d2 (backhome for Popeye) and therefore the black King is not in check. On the other hand, WinChloe considers that Sb1 is not forced to play Sb1-d2…” (“Je crois que les Blancs ont Cb1-d2 (retour pour Popeye) et donc le Roi noir n’est pas en échec. Par contre, WinChloe considère que Cb1 n’est pas obligé a jouer Cb1-d2…..”)

I’d like to invite our readers to help to find out if Popeye & WinChloe have a different interpretation of some of the conditions used (or of the combination) or different solutions are the result of some bugs. I’ll accept a problem for the tournament in a case of a clear different interpretation. (JV)


Circe Parrain: After a capture, the captured piece is reborn only after another piece of its own side has moved. The line between capturing square and rebirth square is parallel with and of same direction and length as the move of this other piece. Pawns can be reborn on 1st and 8th rank. From their own base rank, they may move one-step; if reborn on the promotion rank, the Pawn at once promotes, the promotion piece being determined by the Pawn side.

Anti-Andernach: A piece (excluding King) changes its color after any non-capturing move. After capture, the piece retains its color. Rooks on a1, h1, a8 and h8 can be used for castling, provided the usual other rules for that move are satisfied. After castling, Rooks do not change color, If White makes a non-capturing move with neutral or halfneutral piece, that piece becomes black and vice versa.

a) If a piece can legally move to the square it occupied in the initial position of the problem, it must move to this back-home square. 
b) Back-home moves are prevalent to the virtual capture of the opponent King by any piece, i.e. “checks are fairies”. 
c) If several back-home moves are legal, the side-on-move chooses which one to play. 
d) The back-home square of a Pawn which is promoted during the solution is the initial square of this Pawn. 
See the link https://juliasfairies.com/articles/back-home-fcondition-ndupont/  for more details on the back-home fairy condition.

No.568 Paul Rãican

original – 08.07.2014

Solutions: (click to show/hide)

white Sc5 black Kb4 Pb6 Sd2

ser-h#17                                      (1+3)
Circe Parrain
Back Home
a) version Popeye 4.67
b) version WinChloe 3.26

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Nikola Predrag
Nikola Predrag
July 9, 2014 04:39

It seems that Winchloe after a color-change doesn’t see the same piece as it was before the change.
Otherwise in b), 17…Qxb3 wouldn’t be a check(mate) due to a mandatory back-home move to b6.
That’s why Popeye show this as a solution if bP is initially on d6 (instead on b6).
But then, there would be a cook with wQb3 and wSc2 mating bKa1 (with dualistic order of moves).

The rule d) “The back-home square of a Pawn which is promoted during the solution is the initial square of this Pawn”, suggests that a transformation of some piece doesn’t affect its back-home square.

So, Winchloe doesn’t look convincing about that logic.

Dmitri Turevski
Dmitri Turevski
July 9, 2014 13:21
Reply to  Nikola Predrag

The rule d) “The back-home square of a Pawn which is promoted during the solution is the initial square of this Pawn”, suggests that a transformation of some piece doesn’t affect its back-home square.

Quite the opposite. “Exceptio probat regulam in casibus non exceptis”.

The existence of the special-case rule d) proves the existence of the implied general rule that transformation of the piece does affect its back home square.

Nikola Predrag
Nikola Predrag
July 9, 2014 14:48

Dmitri, rule d) is not explicitly marked as an exception, so it only tells how to deal with the promotions.
Even if we treat it as an exception, the “implied general rule” would not be about all transformations, but only about the promotions.

Of course, rule d) also doesn’t imply that all transformations are irrelevant concerning the BackHome squares.
The inventor should determine the default logic.
Until then, I try to guess what that logic might be and I wrote that “rule d) SUGGESTS…”, and not IMPLIES.
The initial diagram-position looks as a default to me, similarly as in DiagramCirce.

Dmitri Turevski
Dmitri Turevski
July 9, 2014 15:24
Reply to  Nikola Predrag

“No parking allowed on Sundays” implies that parking is allowed on every other day.

Note that:
1) No “explicit mark of exception” is necessary. Sunday is just a special case of a day, this alone makes it an exception rule.
2) The implied general rule concerns all days, not only Sundays.

If you agree that promotion is just special case of transformation (i believe this is exactly what you suggest in your first comment) – well, there you have it.

>The inventor should determine the default logic.
“Exceptio probat …” is well established legal principle to determine the default logic, like, for centuries. If the inventor meant something different, i’m afraid the wording chosen is very unfortunate.

Nikola Predrag
Nikola Predrag
July 9, 2014 16:47

“Exceptio probat …” is well established legal principle to INTERPRETE the default logic, like, for centuries.
And unfortunately it allows tragic MISS-interpretations and dogmas.
“well established legal principles” may last for millennia and still be absurd or at least imprecise if they are taken as absolute ever-valid rules.

“No parking allowed on Sundays” says nothing about the other days, just as it would be the case with “Parking allowed on Sundays”.
There are some other general rules/principles/customs which determine that the above statements are the exceptions.

“Parking on my private ground is allowed” does not mean that parking is not allowed on the neighboring public ground.

Fairy definitions are rarely absolutely precise for all possible cases. A desirable default logic should be correspondent with (fairy)chess as widely as possible.

So far I see the best correspondence with the logic of DiagramCirce. Certainly not with the concept of rebirth but with the concept of the unchangeable diagram-square of any unit, irrelevantly to its transformation.

Does “Walking on the grass is not allowed” imply that general principle allows everything else, like driving on that grass?
Or the logic suggests to care about the grass?

July 9, 2014 18:09
Reply to  Nikola Predrag

“Unchangeable diagram-square of any unit, irrelevantly to its tranformation”.

This meta-rule seems to always be adopted both by Popeye and WinChloe, with only one exception: WinChloe regarding change of color. This exception has its own inherent logic (the involved piece belongs to the other side, this is very different than being simply equipped with other move possibilities), that’s why I don’t consider it as a bug.

Nevertheless I prefer Popeye’s approach (i.e. to always follow the above meta-rule), which thus might be considered as the default rule.

Dmitri Turevski
Dmitri Turevski
July 9, 2014 19:27
Reply to  dupont

But then after b) 17…Qxb3 the wQ backhome square must be b1, right? And there’s no check.

July 9, 2014 20:10

It seems that after a color change WinChloe considers that the piece is new, and therefore not equipped with a backhome-square (backhome-squares are defined only for pieces present on the diagram position).

For example, after 1. b1=Q(w) Qb1xb3 2…Qb3-b1 is not mandatory, although the Qb3 is still white.

The same is true for the black side, i.e. b2 is no more the home-square of the promoted piece which is again of black color: After 1. b1=Q(w) Qb1-b3 2…Qb3-b2 is not mandatory, although Qb3 is black.

Nikola Predrag
Nikola Predrag
July 9, 2014 19:37
Reply to  dupont

Hm, the diagram-square in DiagramCirce is not affected by the change of color and belonging to the other side. This looks as a pretty simple and consistent logic.

Winchloe complicates the BackHome definition but if it could be made clear and precise, it could be accepted as interesting.
What is the BackHome-square of a piece which has changed its color? Does it mean that after the color-change, such piece becomes free, without a BackHome square?

What happens with that wSb1, the definition mentions only the “initial position of the problem”.
If an “adjusted” definition would treat wSb1 as a new entity with its BackHome square b1, then another color-change after wSb1-a3=bS, would mean another new entity with its BackHome square a3.

Without a proper adjustment of BackHome definition, it looks more like a bug.

July 9, 2014 20:18
Reply to  Nikola Predrag

Oups, crossed messages… Dmitri is right, for WinChloe a piece with new color is new, and hence is not equipped with a backhome square (this follows the definition, only the pieces present on the diagram position have a back-home square).

Dmitri Turevski
Dmitri Turevski
July 10, 2014 08:27
Reply to  dupont

It could be also worth noting why Winchloe does not find the solution of a)

The key position seems to be after a) … 16.Qa3 [a3=w], in Winchloe no white backhome move is possible here, so this is illegal selfcheck. In Popeye backhome move Sb1-d2 rules the check out.

Nikola Predrag
Nikola Predrag
July 10, 2014 10:48

I think that in the Popeye’s solution a) Winchloe would consider the moves 4,6,8,10 and 16 as selfchecks.

Dmitri Turevski
Dmitri Turevski
July 10, 2014 12:17

Yes, you are correct, Nikola.

Dmitri Turevski
Dmitri Turevski
July 9, 2014 19:38
Reply to  dupont

Disregard that. Apparently, after the change of color Winchloe considers the new piece as not having a backhome square at all.

Dmitri Turevski
Dmitri Turevski
July 9, 2014 19:24
Reply to  Nikola Predrag

Logic suggests that “Walking on the grass is not allowed” is a very poor choice of words if you care about grass.

Because it definitely says that the grass is poisoneus, walking is not allowed (but it’s ok to drive) 🙂

You have a very good point about consistency of fairy chess and analogy of Backhome to DiagramCirce, but programmers still must follow the definition to the letter. And letter d) suggests that promotion must be treated as a special case 🙂

Nikola Predrag
Nikola Predrag
July 9, 2014 19:54

Letter d) doesn’t state that promotion is a special case!

A creation of some rule quite naturally implies that previously there was NO “general principle”. Such rule might initiate a creation of “general principle”.

Only if the exceptionality of a rule is mentioned, that would imply the existence of a different default principle.

July 9, 2014 12:17

Yes Nikola, it seems things are going exactly as you said. A simple experience can be made with only one officer on the board and conditions Antiandernach & Backhome.

Then WinChloe (under its “mode rédaction”, very useful to detect legal moves) is accepting each possible series. It means that after a color-change the “new” piece is no more equipped with its backhome square. Even after 2 color-changes, although the “new” piece is now of same color than in the beginning, its backhome square is not the original one.

There is no “redaction mode” with Popeye, but you can nevertheless construct an easy test, for example by putting only a black knight on a1 and, as stipulation, an help sequence in 1 move, with target c5 as goal. Popeye will then provide no solution as, after 1.Sb3 (w) the only Popeye-legal move is the backhome move 2.Sa1(b). On the contrary, WinChloe is accepting 2.Sc5(b).

Like you Nikola, I think that’s Popeye’s logic is here more in touch with the spirit of the Backhome rules. Nevertheless I don’t think that WinChloe’s approach is a bug so, to my eyes, Paul’s problem is sound (this remark is for you Julia!).

Another interesting possibility is to look at how Popeye handles Einstein & Backhome. As an example:

Cond einstein
Cond backhome
Stip ser-zb42
Pieces white Qa1

1.Qa1-d4=R 2.Rd4-b4=B z
1.Qa1-b2=R 2.Rb2-b4=B z

Note that 1.Qa4=R 2.Rb4=B is not Popeye-valid. The only possible reason I can see is that, from a4, the rook has a legal backhome move Ra1=B.

So Popeye’s “philosophy” is that a piece is not “new” after any change of color/type, but is obliged to go to its backhome square only if this move is legal in the orthodox sense. Indeed, in the above first solution, d4=R can’t reach the backhome square a1, hence is free to move to the target b4.

Nikola Predrag
Nikola Predrag
July 9, 2014 13:59

The question is about any piece which was anyhow transformed during the play.
Is it the same piece or not?
If not, then a promoted Pawn is also a new piece with a new BackHome square (square of promotion).

Winchloe’s “type” of Back Home should treat all transformations in the same way, including the promotions.

We know the Circe-rebirth squares for all pieces on a diagram. But as the pieces move, their rebirth square will depend on their “position and shape/state” in the moment of capture.

DiagramCirce “remembers” the diagram-positions of the pieces, irrelevantly to their transformations and changed positions. Example:
White chameleon Sb8
White chameleon Sa7
Black Ka8 Pc7 Pb6
Stipulation H#2
Condition AntiAndernachChess DiagramCirce
1.c7-c5=w cSa7-c6=cB=b 2.b6*c5[+wPc7] cSb8*c6=cB[+bcBa7]#
1.c7-c6=w cSa7-b5=cB=b 2.cBb5*c6=cR[+wPc7] cSb8*c6=cB[+bcRa7]#

BackHome’s logic is similar to DiagramCirce – “remembering” the diagram position, irrelevantly to the changes caused by play.

July 9, 2014 15:06
Reply to  Nikola Predrag

I don’t consider a back-home move to be a rebirth, the main difference being that the first one is a “move” (i.e. must be conform to the moving possibilities of the piece under consideration), while the second is not.

As a matter of fact WinChloe and Popeye seem to agree when a piece changes its type (not its color). As an example with wQa1, direct series in Einstein & Back-home, 1.Qb2=R 2.Rc2=B 3.Bb3=S is sound, but now 4.Sa1 is the only legal move.

I like this rule: the back-home square a1 is memorized, Rb2 and Bc2 can’t reach this spot (no rebirth!), but Sb3 can reach it via a Knight move, and hence is obliged to reach it, due to the back-home constraint.

Would love your thoughts, please comment.x