P.S.
If you have to simply sum some accounts then:
For DM package the alternative script can be used (using WHEN/ENDWHEN accumulation):
*XDIM_MEMBERSET CATEGORY = BUDGET
*XDIM_MEMBERSET ACCOUNT= B,C
*WHEN ACCOUNT
*IS *
*REC(EXPRESSION = %VALUE%,ACCOUNT = "A")
*ENDWHEN
Vadim