json_to_prolog help

Hello,

I am having some difficulties with the following MWE.  I'm new to
Prolog and this is a small part of a prototype which will, hopefully,
make Prolog a significant part of our production system!  I am
uncertain if my approach is fundamentally flawed (not using the
library correctly), if there is a error, or if I am running up on a
limitation of the JSON support.  I am using pl-6.2.6 which I
downloaded and built.

Here, I am working on code to parse JSON.  The problem is that, in my
example, it does not return a reading record but a json record (may
not be the proper terminology).

:- use_module(library(http/json)).
:- use_module(library(http/json_convert)).
:- json_object
datum(raw:integer, eng:float).
:- json_object
reading_error(sensor_id:integer, rply_code:integer).
:- json_object
reading_datum(sensor_id:integer, data:datum/2).
:- json_object
reading(passwd:integer, datetime:integer, data:list(reading_datum/2)).

Here is the result of two illustrative queries:

?- prolog_to_json(reading(1234,41234,[reading_datum(1,datum(2,3.3))]),JSON_Obj),
format(user_output, '~w', JSON_Obj).
json([passwd=1234,datetime=41234,data=[json([sensor_id=1,data=json([raw=2,eng=3.3])])]])

?- json_to_prolog(json([passwd=1234,datetime=41234,data=[json([sensor_id=1,data=json([raw=2,eng=3.3])])]]),PRLG_Obj).
PRLG_Obj = json([passwd=1234, datetime=41234, data=[reading_datum(1,
datum(2, 3.3))]]).

I want PRLG_Obj to be reading(...), not json(....).

Any help over this hump would be greatly appreciated.  Thank you in advance.

- dan elliott