[Retros] Favour/En Passant

Guus Rol grol33 at gmail.com
Mon Jun 16 05:32:40 EDT 2014

Dear Jonathan,

Excellent case and a real issue. There are many similar situations,
particularly in fairy land like with the "fuddled men" in Turnbulls
infamous article. I had preseved the treatment of these cases for an
"advanced topics" post but I can outline it here. You could have made
things a little worse by making sure that white was "stalemated" apart from
playing the e.p. moves. Deciding that white is stalemated would be illegal
as there would exist no proof game leading to stalemate. Clearly some e.p.
move must therefore be permitted.

The reduction principle on the DGCs is based on the premise that some
preferred option for play remains but such is not always the case. Besides
that, there is the possibility of a "group right", a right that cannot be
proved for each individual member of the group but van be proved to exist
somewhere in the group. The handling of the cases occurs on a higher level
than the handling of the DGC-set / Game set.

*The natural approach is the temporary promotion of all "secondary rights"
to "primary rights" (I am not sure about this terminology yet but you know
what I mean)  in a "rights group" allowing each one the be executed as such
- i.e. as if it were a right to castle. After this promotion, the reduction
from DGC to Game resumes with the modified "rights".*

One issue remains and is reflected by your example. Do you wish to allow
"promotion" only when no other playing option remains or do you wish to
allow it whenever a group right exists  - as in your example? There is an
aspect of personal taste in this choice, but also one of best workability.
Having contemplated this for a while on the basis of fairy forms, my gut
feeling is that it is best to stick with the first choice. Which means that
white cannot play e.p. in the example you presented.but he could if he were
stalemated (or mated) otherwise. But you are entitled to disagree since the
choice steps outside the necessities of a a sound and consistent
decisioning system.

Note: On top of the retro-decisionig-tree is the 1st command: *There must
always be a proof game*.

Best wishes, Guus Rol.

On Mon, Jun 16, 2014 at 10:22 AM, A J Mestel <ajm8 at maths.cam.ac.uk> wrote:

> Now that I'm connected again, I'll repost something I wrote a month ago.
> It feels a bit like a poor joke, which had some point at the time, but
> loses everything in the re-telling, but still, here it is/was.
>     Jonathan
> Date: Fri, 6 Jun 2014 11:04:56 +0100 (BST)
> From: A J Mestel <ajm8 at maths.cam.ac.uk>
> To: The Retrograde Analysis Mailing List <retros at janko.at>
> Subject: Re: En passant
> Did this ever get posted in the list? I never saw it, and got no replies.
> It's a bit dated now, but someone said that en passant was only legal if
> you could prove what the previous move had to be etc. Needs to be a bit
> more precise.
>   Jonathan
> On Wed, 28 May 2014, A J Mestel wrote:
> Someone must have done this before, but consider:
>> W: Kc5 Rd8 Bc8 Nc6 a5 a6 e5 e6
>> B: Kc7 Bb8 b5 d5
>> Can White mate in one? Not according to the definition I read here a few
>> mails ago.
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