public static Document newPartialDocument(DocumentType docType, DocumentId docId, Field field, ValueUpdate update) {
        Document doc = new Document(docType, docId);
        applyUpdate(field, update, doc);
        return doc;
    }
Returns true if this update can be represented as a set of document field values.
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/FieldUpdateHelper.java