[Retros] Variables

[Rol, Guus]

That makes things more interesting Joost. There is more to Stephens comments than just the "Variable Piece". He also adjusts the notation of moves by writing down "Vc4-b3" instead of "Vc4xb3" or "Vc4xb3 e.p." as would be usual. That introduces new unknowns (captures) besides the Variable Piece. One would need to peel off the layers to see if there is anything AP hidden inside (which I doubt). Note that the e.p. capture needs no justification since the preceding double step move is a 100% certainty. The playing options of a Pawn are therefore as clearly defined as those of any other piece and need no justification.

By the way, I've encountered PbyD (prove it by doing it) before and I've always wondered if that could mean something different from PF-strategy. If it included arbitrary e.p. captures as well, what would they need AP-logic for?


Guus Rol
 


-----Oorspronkelijk bericht-----
Van: retros-bounces at janko.at [mailto:retros-bounces at janko.at] Namens Joost de Heer
Verzonden: maandag 24 november 2008 13:09
Aan: The Retrograde Analysis Mailing List
Onderwerp: Re: [Retros] Variables


> I haven't read the original article so I can't be sure of the role it 

> assigns to the à posteriori attribute, but going by the examples I 

> suspect that the AP-qualification is misused here. The role adaptation 

> of the Variable is actually an exquisite example of Post (F)actum 

> logic.


I don't think Stephen intended to use AP in the same sense as it is used in retros. The paragraph that mentions 'a posteriori' is:

[Diagram B, after moves 1. Vb2-b4 Vc4-b3] "Similarly, if White follows with 2. Ra4, this is legal only if b4 is vacant, so again ep capture must have taken place, and if it wasn't known before, now it is known that b3 must be a bP. This is a posteriori justification, not prove it by doing it."

You're probably right in saying PF is a better name.

Joost