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This night the following occurred to me: I normally write a Diagram definition by first specifying all parameters, and then all pieces. Although some parameters (such as the board size) must be set before placing any pieces, in general there is no need for this ordering, and parameter definitions and piece definitions can be mixed in any order. Furthermore, it is not an offense to specify the same parameter multiple times with different value; each definition simply replaces any previous value, so that the value from the last definition prevails.

You could therefore try the following: start with a line symmetry=none amongst the parameter settings. Then specify all pieces with their locations. Because at the time of their processing the symmetry is set to none, it will not automatically place any mirror images, and you specify the squares of both white and black pieces in any way you want. After defining all pieces, you add a line symmetry=mirror . This will alter the symmetry spec, but this new spec will not be used during setup, which is already done. It will be used during shuffling, though, which occurs after the entire Diagram definition is read.