[Retros] At home proof games in 7.0 moves

[andrew buchanan]

Dear Doctor,

The terminology is so hopeless in this area one can only laugh or
scream. :-) And it is of course a completely doomed endeavour to try to
change the nomenclature. Might as well just live with it.

This is how I think it is...

PG has *2* main different meanings:
(a) a sequence of moves (= "game") to demonstrate that an arrangement
of units is in fact a legal position.
(b) a composition where one has to find the unique game that leads in
exactly X.Y moves to a certain position.

Notes:
(1) under definition (a) there may be many proof games leading to a
certain position.
(2) under definition (b) there is just one game leading to that
position.

Now SPG is a special type of PG under definition (b). It has *nothing*
to do with definition (a). There are two types of SPG:

(c) SPG without specified number of moves. This is just like a PG
except that the solver has to find the number of moves, which is the
minimum possible in which the position can be reached, and is unique.
(d) SPG in X.Y moves. This is just like a PG except that in addition
the composer asserts that the position cannot be reached in a shorter
number of moves. Of course there is no reason why the composer should
do this: the composition would be sound if defined as PG (b) or SPG in
X.Y (c). However, I find definition (d) more satisfying somehow, and I
guess others do too.

The final nuance is that because type (d) SPGs exist, this means that
these days PG has a variant definition:

(b') Like (b), but in addition the composer implies that the position
*can* be reached in a shorter number of moves (but maybe not uniquely).

Otto: I have to say I found the descriptions of PG & SPG in the Retro
Corner confusing. I suggest just one page, which presents the various
definitions as described above.

Thanks & regards,
Andrew.

--- DoctorSBD at aol.com wrote:


> Francois> SPG in 7.0 moves, how many solutions?

> 

> I was confused by that anyway, since I always thought SPG meant there

> was only one solution, as opposed to PG, which can have many

> solutions.

> 

> SBD

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