DESCRIPTION:

Eccet can use the class and depth fields to store results of
operations that cannot run in place (in situ).
Several filters (all filters whose name begins with FILTB)
use this backing store as the target for their operations.
The backing store can be manipulated with the commands
COPY_GREYTOBACK, COPY_BACKTOGREY, XCHG_GREYANDBACK,
MIX_BACKTOGREY and ERASE_BACK.