public static boolean isFieldValues(Field field, ValueUpdate update) {
        if (update instanceof AssignValueUpdate) return true;
        if (!(update instanceof MapValueUpdate)) return false;

        DataType fieldType = field.getDataType();
        if (!(fieldType instanceof StructuredDataType)) return false;

        field = ((StructuredDataType)fieldType).getField(String.valueOf(update.getValue()));
        if (field == null) return false;

        return isFieldValues(field, ((MapValueUpdate)update).getUpdate());
    }
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