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Subject:

Don Woods's answer is correct. When the edges and corners

are in their solved positions, the total face center twist is zero

modulo 180 degrees. Thus the number of face centers that are

twisted 90 degrees must be even. A proof of this appeared in

Cube-Lovers last year.

A process to twist the U face center 180 degrees without

changing anything else is (U LR UU L'R')^2. This process comes from

Singmaster's book. I sent Cube-Lovers a general procedure for

solving the supergroup in January.

If you want a copy of either of these messages, send a note

to Hoey@CMU-10A.