MemberOf indexing

Hi all, I'm using the memberOf overlay and would like to use it with this kind of search -filter: (memberOf=*ou=foo,dc=it)

I tried with "index memberOf sub", but this kind of indexing is prohibited by OL telling me:

substr index of attribute "memberOf" disallowed

I've seen that in the schema definition of the attribute this attribute is defined as: ( 1.2.840.113556.1.2.102 NAME 'memberOf' DESC 'Group that the entry belongs to' SYNTAX '1.3.6.1.4.1.1466.115.121.1.12' EQUALITY distinguishedNameMatch USAGE dSAOperation X-ORIGIN 'iPlanet Delegated Administrator' )

So only equality matching seems to be allowed, as far as I can understand of course. Is there a way to obtain an index of type sub ("subfinal" in this case)?

Thanks in advance Marco