Using functions in MDX logic script

How can i use  MDX functions like ABS, SIGN etc. in logic scripts?

Actually, i would like to save always positive values for example for revenues.

The script could be like that:

*WHEN ACCOUNT.ACCTYPE

*IS EXP

    *REC(EXPRESSION = ABS(%VALUE%))

*ENDWHEN

*COMMIT

But this gives a validation error "RefferenceError: ABS is not defined".

Maybe there is some other way to save the absolute value or to change sign in case the value is negative?