My guess is that mabs is defined as a macro because
the author of this file thought it would be better
to avoid a "call" and wanted to do an in-line
expansion.
Possible solutions:
1. just change (eval-when (eval compile)
to (eval-when (compile)
OR
2. remove the definition of mabs, assuming it is defined
correctly elsewhere.
I'm not sure which is preferable.
............
RJF