'Return tuple of OID and all sub-types of attribute type specified
in nameoroid.'

def _at2key(self, nameoroid):

    try:
        return self._attrtype2keytuple[nameoroid]
    except KeyError:
        oid = self._s.getoid(ldap.schema.AttributeType, nameoroid)
        l = nameoroid.lower().split(';')
        l[0] = oid
        t = tuple(l)
        self._attrtype2keytuple[nameoroid] = t
        return t
