When I use the "-gm incremental" option without a guide file, the following error occurs in MAP:
"ERROR:Pack:313- Unable to combine the following symbols into a single slice:
MUXF6 symbol (..) MUXF5 symbol (..)"
In ISE 6.x, separate Area Groups are unable to pack logic together (as was allowed in 5.x). In this instance, the F6 and F5 muxes should be packed together, but are not in the same Area Group.
To work around this issue, ensure that logic from wide functions using F5/F6 muxes are part of the same Area Group.