openldap and overlay constraint woes

I am trying to build an LDAP schema with several attribute types that are constrained to specific values.

The OpenLDAP discussion group quickly re-directed me from trying to use the ldapsyntax with X-ENUM, as this appears to be a proprietary extension. They informed me that I needed to use the slapo-constraint overlay.

The man-page (slapo-constraint(5)) is present. I have set up a simple minimal test case (attached).

The problem I am having is that using slaptest -f slapd.stars.conf -F test.d is yeilding:

overlay "constraint" not found
slaptest: bad configuration directory!

Most of the advice on the net assumes you are compiling openldap; for information assurance reasons we are trying to keep with RHN provided software only.

I have attached three files:
slapd.stars.conf to be placed in /etc/openldap
test.schema to be placed in /etc/openldap/schema
overlay.schema to be placed in /etc/openldap/schema

To duplicate:
mkdir /etc/openldap/test.d
cd /etc/openldap
slaptest -f ./slapd.stars.conf -F test.d