@Override
    public void write(FieldBase field, StructuredFieldValue value) {
        serializeStructuredField(this, generator, field, value);
    }
This method will only be called if there is some type which is not
properly supported in the API, or if something has been changed without
updating this class. This implementation throws an exception if it is
reached.

@throws UnsupportedOperationException if invoked
document/src/main/java/com/yahoo/document/json/JsonWriter.java