[Retros] request for real definition of PRA

Rol, Guus G.A.Rol at umcutrecht.nl
Mon Jan 22 12:55:37 EST 2007


Hi Andrew,

I have extensive ideas about this area but after years I am still not
sure that I have addressed every issue. Remarkably I have posed many of
your questions in the area of "retro-strategy" (FIDEX default,no
retro-forks) and not in "Retro-Variants" but the preliminary analysis is
more or less the same. I'm afraid we have to quote Einsteins in this
field "My aim is to make things as simple as possible, but not simpler
than that". And they are not going to be very simple!

The most fundamental interpretation of RV has been described by
Alexander George and he named it CRAC. Since Alexander forgot about
repetition and 50-moves rules I have no choice but to extend one of his
own conclusions: (Alexander) The analysis of course agrees with the
conclusions we reached above ... (3) that castling cannot be a keymove.
(Guus)... (4) that any reversible move (e.g. Be5) cannot be a key move!
By the same logic applied by George I suggest that we might have been on
the brink of a second repetition and that Be5 was just the move to do
it! As an approach that is both fundamental and practical CRAC exits
here though the backdoor of our Retro Convention Hall.

I mention CRAC because it demonstrates that every useful concept of RV
must in some way be a mix of "retro-defaulting" and "retro-splitting". I
suggest in general that splitting is applied between "equally ranked
partners" and defaulting amongst "unequally ranked partners". The
primary ranks are "preferred" and "unpreferred" (better terminology
welcome). Examples are: "preferred": castling right, no e.p. right, no
repetition-draw, "unpreferred": no castling right, automatic 50 moves
draw (if you apply automaticity here), e.p. right. This implies that in
a fight between "e.p. right" and "castling right" you choose the default
"castling right". But in a fight between "e.p. right" and "no castling
right" you split for RV.

This is somewhat different to your design in the sense that I do not
compare "game states variables" but "the ranks of game state variables".

I know this is not a definition of RV, but I guess we should first
attempt to establish agreement on the content description of RV before
moving into formalizing it.

Guus Rol.





> -----Oorspronkelijk bericht-----

> Van: retros-bounces at janko.at [mailto:retros-bounces at janko.at]

> Namens andrew buchanan

> Verzonden: vrijdag 19 januari 2007 17:43

> Aan: Retros Mailing List

> Onderwerp: [Retros] request for real definition of PRA

>

>

> Dear Retro chums,

>

> I would really appreciate a *definition* of PRA (aka RV, aka

> "Ceriani Ethics").

>

> It's all very well to hear "well it sort of works like this

> in *this* problem". I get a hint of a whiff what it might be

> about. But that gives me little predictive power when it

> comes to the next PRA problem. So I thought I would send this

> email once and then I will know. Hopefully.

>

> Here is my best guess so far as to what PRA might be.

>

> Suppose we have a position. There are a number of game state

> variables (who has the move, which castlings rights are

> retained, what en passants might be on). These variables give

> a set of vectors, e.g. they might be:

>

> W,C1Y,C2Y,E1N

> W,C1Y,C2N,E1N

> B,C1N,C2N,E1Y

> ...

>

> where the first one means there is a proof game which results

> in the position, with White to play, both castlings OK, but

> en passant not working. Etc.

>

> Now some subset of this set is the twinset. How do we pick

> them? I think we define a relation ">" (pronounced

> "dominates") on the vectors. We say that u>v if every piece

> in v has all its powers (and possibly

> more) in u. (So in the list above, the first vector dominates the

> second.)

>

> The PRA twins are those vectors which are not dominated by

> any other. So in the above list it would be the first & third vectors.

>

> Am I right so far?

>

> Note that this definition of PRA is independent of words like

> "castling" & "en passant". Hence it can be applied to fairy

> chess with state as well.

>

> My questions are:

>

> (1) What about "who has the move"? Is that really just

> another element of the vector (although u>v only if the same

> player has the move)? In the (very vaguely described) example

> in the Retro Corner, there is no consideration of the

> possibility that Black has the move. But an email I got

> tonight from someone else says that "the move" is something

> which can vary between PRA twins. I never knew this before.

> Well, obviously it can't be so in the Langstaff example.

> Black can evade mate with e.g. 1...Rf8. Nor was there ever

> the intention that this should happen in Langstaff. So what gives?

>

> (2) What are the retro consequences of moving from the

> overall position to considering a twin? Are we allowed to

> assume that the proof game must be one which results in the

> dominant vector? [This is in contrast to what I believe (but

> I've never seen written down anywhere, that for the

> conventional castling & en passant conventions, there are no

> retro implications. You are *permitted* or *not* *permitted*

> to castle or ep in the forward game, but the game state for

> that position is not altered.] Can someone please confirm?

>

> If it's sometimes one and sometimes the other, then the notation

> *needs* to show it. Just as Madrasi Rex Inclusiv & Exclusiv

> would make absolutely precise which version of Madrasi we are

> talking about.

>

> (3) And for castling, are we looking at actual castling

> rights (or practical ability in the current position, given

> blocking pieces and attacks)? I presume that it is the

> former. [And I think is in common with the conventional conventions.]

>

> Why with all these mega math/legal brains in retro, why are

> the definitions of the conventions so allusive, vague & murky

> everywhere? And in the end, the best hints I've ever got were

> from a professional *hillwalker*! (Ronald Turnbull). Do we

> want to create even greater barriers to entry to our tiny

> hobby? Or are there books I should be reading which contain

> the truth, and I just haven't read them?

>

> If someone can correct my errors here, then I would be happy

> to turn it into a page for the Retro Corner Glossary.

>

> Thanks,

> Andrew.

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