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MatPlus.Net Forum General Question about validity of a composition
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(1) Posted by Bob Baker [Wednesday, Jan 8, 2020 08:42]

Question about validity of a composition

In a particular legal position, each possible history of the position gives White a mate in 2 with a unique key. But one set of histories has one key, and another set of histories has a different key. The position does not make it possible to rule out either set of histories, so it is not possible to specify a unique key as the solution. Is there a name for such a position? Is such a position considered a valid mate in 2 composition? If so, what is the normal way to specify the solution?
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(2) Posted by Hauke Reddmann [Wednesday, Jan 8, 2020 10:34]

That's almost orthodox :-) (I think the first problem
with e.p + OO was pre-1900!) See also here:

You give the variants as if the position were a twin.
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(3) Posted by Joost de Heer [Wednesday, Jan 8, 2020 11:36]

The official name is Retro-Variants. See e.g.
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(4) Posted by Bob Baker [Wednesday, Jan 8, 2020 15:38]

Thanks for the information.
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(5) Posted by shankar ram [Wednesday, Jan 8, 2020 17:22]

RV is the new name for PRA.
A Rose by... ;-)
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(6) Posted by Andrew Buchanan [Wednesday, Jan 8, 2020 17:54]

Dear retro-friends and others,

If you look at the links given above, it correctly states that they are outdated. These introductory articles in Retro Corner were never tidied up properly following Werner Keym's changes to the Codex in 2009.

Really if you want to find out how this all works, the best location is his authoritative (but advanced) article This is particularly true since the current Codex coverage of the matter is quite opaque unless you already know what it's trying to say.

The introductory articles really need to be tidied up. It is essential to remove unnecessary barriers to newcomers entering this difficult but rewarding hobby. I would be happy to redraft and complete those introductory articles so that they align with Werner's. Or maybe Werner would like to tackle these himself? But in either case we need the active support/buy-in of one of the current editors of the Retro Corner.

Please let me know if I can help here.
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MatPlus.Net Forum General Question about validity of a composition