look for a a node type that includes mix:referenceable but isn't any
of the known internal nodetypes that ev. cannot be created through a
session-import
*/
String referenceableNt = null;
NodeTypeIterator it = superuser.getWorkspace().getNodeTypeManager().getPrimaryNodeTypes();
while (it.hasNext() && referenceableNt == null) {
NodeType nt = it.nextNodeType();
String ntName = nt.getName();
if (nt.isNodeType(mixReferenceable) &&
!nt.isAbstract() &&
// TODO: improve....
// ignore are built-in nodetypes (mostly version related)