if (protoNetwork2 == null) {
throw new InvalidArgument("protoNetwork2", protoNetwork2);
}
DocumentTable dt = protoNetwork2.getDocumentTable();
for (DocumentHeader dh : dt.getDocumentHeaders()) {
// add new document header
int did = protoNetwork1.getDocumentTable().addDocumentHeader(dh);
// union namespaces
NamespaceTable nt = protoNetwork2.getNamespaceTable();