Mario Richter mri_two at t-online.de
Wed Oct 13 17:02:19 EDT 2004

Ivan>> Here is Nikola Predrag's retro problem with twins which will be
publish as original on the site www.problemonline.com
Ivan>>
Ivan>> 4r3/1Q5R/1b1P4/4ppR1/8/2p5/2K4P/7k (6+6)
Ivan>> -1& #1 Last 2 halfmoves? ....... is this correct?
Ivan>> a) diagram b) Ph2→h5 c) Qb7→a8 d) -Pe5
Ivan>>
Ivan>> We are not sure how te write the stipulation. The solver has to
choose how to takeback white halfmove and
Ivan>> black halfmove and then white has mate in one.

Joost> I prefer 'Retract <n> halfmoves for a #x'.

Some journals use something like (-1w,-1b)#1, which reads "white retracts 1
move, black retracts 1 move;
white begins to retract (indicated by -1w preceeding -1s); in the following
forward play white mates in one".

Joost> There's something wrong with this composition: since white has to
move after the retractions,
Joost> black just made the last move. This is impossible, because he's in
check.
Joost> Or does who has the move change after the retractions? (white
retracts, black retracts, and then white mates)

Part c) and d) would have a unique solution if the stipulation is:
White takes back his last move, then mates in one.

With that stipulation I can see solutions for part a) and b) only, if the
e.p. capture in the forward play has not be proved legal.

So maybe the intended stipulation is something like:
"After retracting his last move, white is able to mate in one.
What was the last black move before the retracted white move
to make this possible?"

mario