The Retrograde Analysis Corner
Glossary
The Imitator (invented by Th. Kok in 1939) is a fairy piece depicted as a big black spot, and denoted by "I". It does not move by itself, but it imitates every move by normal pieces. Moves that cannot be imitated are illegal, so that the Imitator acts as a constraint on possible moves.
The Imitator does not belong to any side. It cannot be captured.
Let's hope a simple example will make this clear. Assume we start from the following position (why not?):
(Imitator h4)
Here a legal game might start with 1. e3 (Ih5) h5 (Ih3) 2. Bc4 (Ie6) a5 (Ie4) 3. Bxf7 (Ih7).
Now 3 ... Kxf7 is forbidden because the Ih7 cannot follow. Anyway, the bKe8 is not in check, because the Ih7 cannot imitate the Bf7xKe8 "move" (an Imitator does not capture). You are now ready for a little reader's exercice: list black legal moves.
Observe that the Imitator moves together with the piece it imitates. In the sample game we just saw, 1 ... h7-h5 does not bump into the Ih5. Castling is imitated by decomposing into a King move followed by a Rook move.
In retros with an Imitator, there is no fixed square where the I has to start the game. It can be anywhere on the 3rd to 6th row, except when the stipulation says otherwise. (But observe that you won't go far starting with an I on d6 :-)
Now, you can try your hand at a few real retros with an Imitator.
