public static Document newPartialDocument(DocumentId docId, FieldPathUpdate update) {
        Document doc = new Document(update.getDocumentType(), docId);
        applyUpdate(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/FieldPathUpdateHelper.java