Group configuration to limit total number of memberships

There are times that limiting the number of members in a group has value.

It would be helpful to have data driven (attribute based) limits that could control the
       subject source(es)
       and number ( MAX and MIN)  of subjects from those sources for a given group.

      and a "only subject sources" list too.

Suggested: Throw a veto of the membership add when a violation happens.
     Advanced option: Make the veto configurable to a "warning" and send email instead of a veto.

Example uses:
     MIN = 2    --> prevent having a group with only one member. Useful for "admin" groups.
     MAX = 50  --> useful for limiting due to licensing or other external limits on a provisioned service

     source "g:isa" could be excluded ( by setting MAX =0 ) if nesting groups are disallowed.
     source "g:gsa", could be excluded ( by setting MAX =0 ) if nesting groups are disallowed.
     source "grouperEntities", could be excluded ( by setting MAX =0 ) if grouper Entities are disallowed.

etc....

I picture a set of attributes like:
   groupLimits
           sourcesAllowed =  "gisa,g:gsa,jdbc"    // only allow these three sources as members
           sourceMaxLimit  =  "g:isa=0"              // limit direct and indirect membership adds
           sourceMaxLimit  =  "g:gsa=0"
           sourceMaxLimit  =  "jdbc=50"

NOTE: if multiple subject sources need to be limited, then having "ref groups" ( that are each limited to a smaller number)  that are embedded in other ref groups with the total max enforced there.