FILTEREVENTS inherit & extend

Currently if FILTEREVENTS is defined on a child group it overrides the parent group's value of FILTEREVENTS.  I would like to have the option to make child groups inherit the parent's value but concatenate FILTEREVENTS with additional values just for the child group.

For example:

Parent group has FILTEREVENTS = 10|20|30

Child group has FILTEREVENTS = 40|50|60

Resulting child group FILTEREVENTS = 10|20|30|40|50|60