[j-nsp] any way to do group inheritence only if parent exists?

[Chuck Anderson]

Is there any way to inherit a configuration group setting, but only if
the parent object already exists?  For example, if I apply this:

set groups ND6 interfaces irb unit <*> family inet6 nd6-stale-time 600
set interfaces apply-groups ND6

then all irb interfaces get a "family inet6" with link-local
addressing created and the nd6-state-time setting applied.  How do I
inherit the nd6-stale-time setting only if there is already a
configured "family inet6" so I don't get IPv6 link-locals on IRBs
where I only want IPv4?