The Retrograde Analysis Corner
Glossary
Fairy retros are retros with fairy chessmen and/or rules.
Many have been composed. The motivations for introducing fairy pieces and/or conditions in retros are exactly as for usual forward problems.
It is quite straightforward to understand how fairy conditions can be incorporated in retros once you see that all retro-stipulations can be rephrased in terms of the existence of legal proof games with given peculiarities. Then, assuming a given fairy condition, you just have to know what is a legal proof game.
This is no problem in many cases where you just play the "usual" fairy moves starting from the usual starting position. This applies e.g. to Circe chess, Monochromatic chess, chess on a grid-board , or on a vertical cylinder.
Some other fairy conditions needs explicit stipulations when the implicit starting position will not do. But there are some default rules: most used is the following rule "when fairy men appear on the diagram, it is assumed that they did not exist in the starting position but that promotions in these fairy men are allowed".
Sometimes, the stipulation says that the fairy men replace (some of) the usual men: e.g. Maos or Nightriders instead of Knights. (This situation is more properly seen as a fairy condition because it assumes that the usual Knights do not exist anymore: you can't promote in Knight, ...)
Observe that in some retraction problems with a stipulation of the form "White retracts his last move and then mate in one" the legality of the position is not really at stake, and the only question is to find the hidden tricky retraction move that will allow a mate in one. These problems will accomodate fairy men on e.g. an horizontal cylinder without requiring specific explanations about how the game really started.
The retro-glossary has entries for a few fairy conditions / fairy men, most of them with actual examples of how they can be used to achieve specific effects in retros. Look at them! I also started to collect some fairy retros on a page of their own.
It is instructive to reflect on the fact that many of the most popular fairy rules used in retros mostly restrict the list of legal moves in a given position. These constraints on valid proof games let the composer enforce additional maneuvers in the retro-play without requiring the many apparent scars that are customary.
