* @throws InvalidQueryException if one of the selector node types is
* unknown.
*/
private void checkNodeTypes() throws InvalidQueryException {
try {
qomTree.accept(new DefaultTraversingQOMTreeVisitor() {
public Object visit(SelectorImpl node, Object data) throws Exception {
String ntName = node.getNodeTypeName();
if (!session.getNodeTypeManager().hasNodeType(ntName)) {
throw new Exception(ntName + " is not a known node type");
}