packages/RDF/rdf_write.pl

/*  Part of SWI-Prolog

    Author:        Jan Wielemaker
    E-mail:        J.Wielemaker@vu.nl
    WWW:           http://www.swi-prolog.org
    Copyright (c)  2010-2020, University of Amsterdam
                              CWI, Amsterdam
    All rights reserved.

    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions
    are met:

    1. Redistributions of source code must retain the above copyright
       notice, this list of conditions and the following disclaimer.

    2. Redistributions in binary form must reproduce the above copyright
       notice, this list of conditions and the following disclaimer in
       the documentation and/or other materials provided with the
       distribution.

    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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*/

:- module(rdf_write,
          [ rdf_write_xml/2             % +Stream, +Triples
          ]).
:- autoload(library(assoc),
	    [empty_assoc/1,put_assoc/4,assoc_to_keys/2,get_assoc/3]).
:- autoload(library(debug),[assertion/1]).
:- autoload(library(lists),[member/2,append/3,select/3]).
:- autoload(library(sgml),
	    [xml_quote_attribute/3, xml_name/1, xml_quote_cdata/3, xml_is_dom/1]).
:- autoload(library(sgml_write),[xml_write/3]).
:- autoload(library(semweb/rdf_db),
	    [rdf_global_id/2, rdf_register_ns/2, rdf_is_bnode/1, rdf_equal/2]).


/** <module> Write RDF/XML from a list of triples

This module writes an RDF/XML document  from   a  list of triples of the
format rdf(Subject, Predicate, Object).  It   is  primarily intended for
communicating computed RDF model fragments   to  external programs using
RDF/XML.

When used from the HTTP library, use the following code:

==
reply_graph(RDF) :-
        format('Content-type: application/rdf+xml; charset=UTF-8~n~n'),
        rdf_write_xml(current_output, RDF).
==

@author Jan Wielemaker
@see    library(semweb/rdf_db) offers saving a named graph directly from
        the RDF database.
*/


                 /*******************************
                 *           WRITE RDFXML       *
                 *******************************/

%!  rdf_write_xml(+Out:stream, +Triples:list(rdf(S,P,O))) is det.
%
%   Write an RDF/XML serialization of Triples to Out.

rdf_write_xml(Out, Triples) :-
    sort(Triples, Unique),
    rdf_write_header(Out, Unique),
    node_id_map(Unique, AnonIDs),
    rdf_write_triples(Unique, AnonIDs, Out),
    rdf_write_footer(Out).


                 /*******************************
                 *        HEADER/FOOTER         *
                 *******************************/

%!  rdf_write_header(+Out, +Triples)
%
%   Save XML document header, doctype and open the RDF environment.
%   This predicate also sets up the namespace notation.

rdf_write_header(Out, Triples) :-
    xml_encoding(Out, Enc, Encoding),
    format(Out, '<?xml version=\'1.0\' encoding=\'~w\'?>~n', [Encoding]),
    format(Out, '<!DOCTYPE rdf:RDF [', []),
    used_namespaces(Triples, NSList),
    (   member(Id, NSList),
        ns(Id, NS),
        xml_quote_attribute(NS, NSText0, Enc),
        xml_escape_parameter_entity(NSText0, NSText),
        format(Out, '~N    <!ENTITY ~w \'~w\'>', [Id, NSText]),
        fail
    ;   true
    ),
    format(Out, '~N]>~n~n', []),
    format(Out, '<rdf:RDF', []),
    (   member(Id, NSList),
        format(Out, '~N    xmlns:~w="&~w;"~n', [Id, Id]),
        fail
    ;   true
    ),
    format(Out, '>~n', []).


xml_encoding(Out, Enc, Encoding) :-
    stream_property(Out, encoding(Enc)),
    (   xml_encoding_name(Enc, Encoding)
    ->  true
    ;   throw(error(domain_error(rdf_encoding, Enc), _))
    ).

xml_encoding_name(ascii,       'US-ASCII').
xml_encoding_name(iso_latin_1, 'ISO-8859-1').
xml_encoding_name(utf8,        'UTF-8').

%!  xml_escape_parameter_entity(+In, -Out) is det.
%
%   Escape % as &#37; for entity declarations.

xml_escape_parameter_entity(In, Out) :-
    sub_atom(In, _, _, _, '%'),
    !,
    atom_codes(In, Codes),
    phrase(escape_parent(Codes), OutCodes),
    atom_codes(Out, OutCodes).
xml_escape_parameter_entity(In, In).

escape_parent([]) --> [].
escape_parent([H|T]) -->
    (   { H == 37 }
    ->  "&#37;"
    ;   [H]
    ),
    escape_parent(T).

%!  used_namespaces(+Triples:list(rdf(S,P,O)), -List:atom) is det.
%
%   Return the list of namespace abbreviations used in a set of
%   triples.

used_namespaces(Triples, NSList) :-
    decl_used_predicate_ns(Triples),
    resources(Triples, Resources),
    empty_assoc(A0),
    put_assoc(rdf, A0, *, A1),      % needed for rdf:RDF
    res_used_namespaces(Resources, _NoNS, A1, A),
    assoc_to_keys(A, NSList).


res_used_namespaces([], [], A, A).
res_used_namespaces([Resource|T], NoNS, A0, A) :-
    ns(NS, Full),
    Full \== '',
    atom_concat(Full, Local, Resource),
    xml_name(Local),
    !,
    put_assoc(NS, A0, *, A1),
    res_used_namespaces(T, NoNS, A1, A).
res_used_namespaces([R|T0], [R|T], A0, A) :-
    res_used_namespaces(T0, T, A0, A).

%!  resources(+Triples:list(rdf(S,P,O)), -Resources:list(atom)) is det.
%
%   Resources is the set of resources referenced in Triples.

resources(Triples, Resources) :-
    phrase(resources(Triples), Raw),
    sort(Raw, Resources).

resources([]) -->
    [].
resources([rdf(S,P,O)|T]) -->
    [S,P],
    object_resources(O),
    resources(T).

object_resources(Atom) -->
    { atom(Atom) },
    !,
    [ Atom ].
object_resources(literal(type(Type, _))) -->
    !,
    [ Type ].
object_resources(_) -->
    [].

%!  decl_used_predicate_ns(+Triples:list(rdf(S,P,O)))
%
%   For every URL used as a predicate   we *MUST* define a namespace
%   as we cannot use names holding /, :, etc. as XML identifiers.

:- thread_local
    predicate_ns/2.

decl_used_predicate_ns(Triples) :-
    retractall(predicate_ns(_,_)),
    (   member(rdf(_,P,_), Triples),
        decl_predicate_ns(P),
        fail
    ;   true
    ).

decl_predicate_ns(Pred) :-
    predicate_ns(Pred, _),
    !.
decl_predicate_ns(Pred) :-
    rdf_global_id(NS:Local, Pred),
    xml_name(Local),
    !,
    assert(predicate_ns(Pred, NS)).
decl_predicate_ns(Pred) :-
    is_bag_li_predicate(Pred),
    !.
decl_predicate_ns(Pred) :-
    atom_codes(Pred, Codes),
    append(NSCodes, LocalCodes, Codes),
    xml_codes(LocalCodes),
    !,
    (   NSCodes \== []
    ->  atom_codes(NS, NSCodes),
        (   ns(Id, NS)
        ->  assert(predicate_ns(Pred, Id))
        ;   between(1, infinite, N),
            atom_concat(ns, N, Id),
            \+ ns(Id, _)
        ->  rdf_register_ns(Id, NS),
            print_message(informational,
                          rdf(using_namespace(Id, NS)))
        ),
        assert(predicate_ns(Pred, Id))
    ;   assert(predicate_ns(Pred, -)) % no namespace used
    ).

xml_codes([]).
xml_codes([H|T]) :-
    xml_code(H),
    xml_codes(T).

xml_code(X) :-
    code_type(X, csym),
    !.
xml_code(0'-).                          % '


rdf_write_footer(Out) :-
    format(Out, '</rdf:RDF>~n', []).


                 /*******************************
                 *          ANONYMOUS IDS       *
                 *******************************/

%!  node_id_map(+Triples, -IdMap) is det.
%
%   Create an assoc Resource -> NodeID for those anonymous resources
%   in Triples that need  a  NodeID.   This  implies  all  anonymous
%   resources that are used multiple times as object value.

node_id_map(Triples, IdMap) :-
    anonymous_objects(Triples, Objs),
    msort(Objs, Sorted),
    empty_assoc(IdMap0),
    nodeid_map(Sorted, 0, IdMap0, IdMap).

anonymous_objects([], []).
anonymous_objects([rdf(_,_,O)|T0], Anon) :-
    rdf_is_bnode(O),
    !,
    Anon = [O|T],
    anonymous_objects(T0, T).
anonymous_objects([_|T0], T) :-
    anonymous_objects(T0, T).

nodeid_map([], _, Map, Map).
nodeid_map([H,H|T0], Id, Map0, Map) :-
    !,
    remove_leading(H, T0, T),
    atom_concat(bn, Id, NodeId),
    put_assoc(H, Map0, NodeId, Map1),
    Id2 is Id + 1,
    nodeid_map(T, Id2, Map1, Map).
nodeid_map([_|T], Id, Map0, Map) :-
    nodeid_map(T, Id, Map0, Map).

remove_leading(H, [H|T0], T) :-
    !,
    remove_leading(H, T0, T).
remove_leading(_, T, T).


                 /*******************************
                 *            TRIPLES           *
                 *******************************/

rdf_write_triples(Triples, NodeIDs, Out) :-
    rdf_write_triples(Triples, NodeIDs, Out, [], Anon),
    rdf_write_anon(Anon, NodeIDs, Out, Anon).

rdf_write_triples([], _, _, Anon, Anon).
rdf_write_triples([H|T0], NodeIDs, Out, Anon0, Anon) :-
    arg(1, H, S),
    subject_triples(S, [H|T0], T, OnSubject),
    (   rdf_is_bnode(S)
    ->  rdf_write_triples(T, NodeIDs, Out, [anon(S,_,OnSubject)|Anon0], Anon)
    ;   rdf_write_subject(OnSubject, S, NodeIDs, Out, Anon0),
        rdf_write_triples(T, NodeIDs, Out, Anon0, Anon)
    ).

subject_triples(S, [H|T0], T, [H|M]) :-
    arg(1, H, S),
    !,
    subject_triples(S, T0, T, M).
subject_triples(_, T, T, []).


rdf_write_anon([], _, _, _).
rdf_write_anon([anon(Subject, Done, Triples)|T], NodeIDs, Out, Anon) :-
    Done \== true,
    !,
    Done = true,
    rdf_write_subject(Triples, Subject, NodeIDs, Out, Anon),
    rdf_write_anon(T, NodeIDs, Out, Anon).
rdf_write_anon([_|T], NodeIDs, Out, Anon) :-
    rdf_write_anon(T, NodeIDs, Out, Anon).

rdf_write_subject(Triples, Subject, NodeIDs, Out, Anon) :-
    rdf_write_subject(Triples, Out, Subject, NodeIDs, -, 0, Anon),
    !,
    format(Out, '~n', []).
rdf_write_subject(_, Subject, _, _, _) :-
    throw(error(rdf_save_failed(Subject), 'Internal error')).

rdf_write_subject(Triples, Out, Subject, NodeIDs, DefNS, Indent, Anon) :-
    rdf_equal(rdf:type, RdfType),
    select(rdf(_, RdfType,Type), Triples, Triples1),
    \+ rdf_is_bnode(Type),
    rdf_id(Type, DefNS, TypeId),
    xml_is_name(TypeId),
    !,
    format(Out, '~*|<', [Indent]),
    rdf_write_id(Out, TypeId),
    save_about(Out, Subject, NodeIDs),
    save_attributes(Triples1, DefNS, Out, NodeIDs, TypeId, Indent, Anon).
rdf_write_subject(Triples, Out, Subject, NodeIDs, _DefNS, Indent, Anon) :-
    format(Out, '~*|<rdf:Description', [Indent]),
    save_about(Out, Subject, NodeIDs),
    save_attributes(Triples, rdf, Out, NodeIDs, rdf:'Description', Indent, Anon).

xml_is_name(_NS:Atom) :-
    !,
    xml_name(Atom).
xml_is_name(Atom) :-
    xml_name(Atom).

save_about(Out, Subject, NodeIDs) :-
    rdf_is_bnode(Subject),
    !,
    (   get_assoc(Subject, NodeIDs, NodeID)
    ->  format(Out,' rdf:nodeID="~w"', [NodeID])
    ;   true
    ).
save_about(Out, Subject, _) :-
    stream_property(Out, encoding(Encoding)),
    rdf_value(Subject, QSubject, Encoding),
    format(Out, ' rdf:about="~w"', [QSubject]),
    !.
save_about(_, _, _) :-
    assertion(fail).

%!  save_attributes(+List, +DefNS, +Out, +NodeIDs, Element, +Indent, +Anon)
%
%   Save the attributes.  Short literal attributes are saved in the
%   tag.  Others as the content of the description element.  The
%   begin tag has already been filled.

save_attributes(Triples, DefNS, Out, NodeIDs, Element, Indent, Anon) :-
    split_attributes(Triples, InTag, InBody),
    SubIndent is Indent + 2,
    save_attributes2(InTag, DefNS, tag, Out, NodeIDs, SubIndent, Anon),
    (   InBody == []
    ->  format(Out, '/>~n', [])
    ;   format(Out, '>~n', []),
        save_attributes2(InBody, _, body, Out, NodeIDs, SubIndent, Anon),
        format(Out, '~N~*|</~w>~n', [Indent, Element])
    ).

%       split_attributes(+Triples, -HeadAttrs, -BodyAttr)
%
%       Split attribute (Name=Value) list into attributes for the head
%       and body. Attributes can only be in the head if they are literal
%       and appear only one time in the attribute list.

split_attributes(Triples, HeadAttr, BodyAttr) :-
    duplicate_attributes(Triples, Dupls, Singles),
    simple_literal_attributes(Singles, HeadAttr, Rest),
    append(Dupls, Rest, BodyAttr).

%       duplicate_attributes(+Attrs, -Duplicates, -Singles)
%
%       Extract attributes that appear more than onces as we cannot
%       dublicate an attribute in the head according to the XML rules.

duplicate_attributes([], [], []).
duplicate_attributes([H|T], Dupls, Singles) :-
    arg(2, H, Name),
    named_attributes(Name, T, D, R),
    D \== [],
    append([H|D], Dupls2, Dupls),
    !,
    duplicate_attributes(R, Dupls2, Singles).
duplicate_attributes([H|T], Dupls2, [H|Singles]) :-
    duplicate_attributes(T, Dupls2, Singles).

named_attributes(_, [], [], []) :- !.
named_attributes(Name, [H|T], D, R) :-
    (   arg(2, H, Name)
    ->  D = [H|DT],
        named_attributes(Name, T, DT, R)
    ;   R = [H|RT],
        named_attributes(Name, T, D, RT)
    ).

%       simple_literal_attributes(+Attributes, -Inline, -Body)
%
%       Split attributes for (literal) attributes to be used in the
%       begin-tag and ones that have to go into the body of the description.

simple_literal_attributes([], [], []).
simple_literal_attributes([H|TA], [H|TI], B) :-
    in_tag_attribute(H),
    !,
    simple_literal_attributes(TA, TI, B).
simple_literal_attributes([H|TA], I, [H|TB]) :-
    simple_literal_attributes(TA, I, TB).

in_tag_attribute(rdf(_,P,literal(Text))) :-
    atom(Text),                     % may not have lang qualifier
    atom_length(Text, Len),
    Len < 60,
    \+ is_bag_li_predicate(P).


%       save_attributes(+List, +DefNS, +TagOrBody, +Out, +NodeIDs, +Indent, +Anon)
%
%       Save a list of attributes.

save_attributes2([], _, _, _, _, _, _).
save_attributes2([H|T], DefNS, Where, Out, NodeIDs, Indent, Anon) :-
    save_attribute(Where, H, DefNS, Out, NodeIDs, Indent, Anon),
    save_attributes2(T, DefNS, Where, Out, NodeIDs, Indent, Anon).

%!  save_attribute(+Where, +Triple, +DefNS, +Out, +NodeIDs, +Indent, +Anon)

save_attribute(tag, rdf(_, Name, literal(Value)), DefNS, Out, _, Indent, _Anon) :-
    AttIndent is Indent + 2,
    rdf_att_id(Name, DefNS, NameText),
    stream_property(Out, encoding(Encoding)),
    xml_quote_attribute(Value, QVal, Encoding),
    format(Out, '~N~*|', [AttIndent]),
    rdf_write_id(Out, NameText),
    format(Out, '="~w"', [QVal]).
save_attribute(body, rdf(_,Name,literal(Literal)), DefNS, Out, _, Indent, _) :-
    !,
    rdf_p_id(Name, DefNS, NameText),
    format(Out, '~N~*|<', [Indent]),
    rdf_write_id(Out, NameText),
    (   Literal = lang(Lang, Value)
    ->  rdf_id(Lang, DefNS, LangText),
        format(Out, ' xml:lang="~w">', [LangText])
    ;   Literal = type(Type, Value)
    ->  (   rdf_equal(Type, rdf:'XMLLiteral')
        ->  write(Out, ' rdf:parseType="Literal">'),
            Value = Literal
        ;   stream_property(Out, encoding(Encoding)),
            rdf_value(Type, QVal, Encoding),
            format(Out, ' rdf:datatype="~w">', [QVal])
        )
    ;   atomic(Literal)
    ->  write(Out, '>'),
        Value = Literal
    ;   write(Out, ' rdf:parseType="Literal">'),
        Value = Literal
    ),
    save_attribute_value(Value, Out, Indent),
    write(Out, '</'), rdf_write_id(Out, NameText), write(Out, '>').
save_attribute(body, rdf(_, Name, Value), DefNS, Out, NodeIDs, Indent, Anon) :-
    rdf_is_bnode(Value),
    !,
    (   memberchk(anon(Value, Done, ValueTriples), Anon)
    ->  true
    ;   ValueTriples = []
    ),
    rdf_p_id(Name, DefNS, NameText),
    format(Out, '~N~*|<', [Indent]),
    rdf_write_id(Out, NameText),
    (   var(Done)
    ->  Done = true,
        SubIndent is Indent + 2,
        (   rdf_equal(RdfType, rdf:type),
            rdf_equal(ListClass, rdf:'List'),
            memberchk(rdf(_, RdfType, ListClass), ValueTriples)
        ->  format(Out, ' rdf:parseType="Collection">~n', []),
            rdf_save_list(ValueTriples, Out, Value, NodeIDs, DefNS, SubIndent, Anon)
        ;   format(Out, '>~n', []),
            rdf_write_subject(ValueTriples, Out, Value, NodeIDs, DefNS, SubIndent, Anon)
        ),
        format(Out, '~N~*|</', [Indent]),
        rdf_write_id(Out, NameText),
        format(Out, '>~n', [])
    ;   get_assoc(Value, NodeIDs, NodeID)
    ->  format(Out, ' rdf:nodeID="~w"/>', [NodeID])
    ;   assertion(fail)
    ).
save_attribute(body, rdf(_, Name, Value), DefNS, Out, _, Indent, _Anon) :-
    stream_property(Out, encoding(Encoding)),
    rdf_value(Value, QVal, Encoding),
    rdf_p_id(Name, DefNS, NameText),
    format(Out, '~N~*|<', [Indent]),
    rdf_write_id(Out, NameText),
    format(Out, ' rdf:resource="~w"/>', [QVal]).

save_attribute_value(Value, Out, _) :-  % strings
    atom(Value),
    !,
    stream_property(Out, encoding(Encoding)),
    xml_quote_cdata(Value, QVal, Encoding),
    write(Out, QVal).
save_attribute_value(Value, Out, _) :-  % numbers
    number(Value),
    !,
    writeq(Out, Value).             % quoted: preserve floats
save_attribute_value(Value, Out, Indent) :-
    xml_is_dom(Value),
    !,
    XMLIndent is Indent+2,
    xml_write(Out, Value,
              [ header(false),
                indent(XMLIndent)
              ]).
save_attribute_value(Value, _Out, _) :-
    throw(error(save_attribute_value(Value), _)).

rdf_save_list(_, _, List, _, _, _, _) :-
    rdf_equal(List, rdf:nil),
    !.
rdf_save_list(ListTriples, Out, List, NodeIDs, DefNS, Indent, Anon) :-
    rdf_equal(RdfFirst, rdf:first),
    memberchk(rdf(List, RdfFirst, First), ListTriples),
    (   rdf_is_bnode(First),
        memberchk(anon(First, true, FirstTriples), Anon)
    ->  nl(Out),
        rdf_write_subject(FirstTriples, Out, First, NodeIDs, DefNS, Indent, Anon)
    ;   stream_property(Out, encoding(Encoding)),
        rdf_value(First, QVal, Encoding),
        format(Out, '~N~*|<rdf:Description about="~w"/>',
               [Indent, QVal])
    ),
    (   rdf_equal(RdfRest, rdf:rest),
        memberchk(rdf(List, RdfRest, List2), ListTriples),
        \+ rdf_equal(List2, rdf:nil),
        memberchk(anon(List2, true, List2Triples), Anon)
    ->  rdf_save_list(List2Triples, Out, List2, NodeIDs, DefNS, Indent, Anon)
    ;   true
    ).

%!  rdf_p_id(+Resource, +DefNS, -NSLocal)
%
%   As rdf_id/3 for predicate names.  Maps _:<N> to rdf:li.
%
%   @tbd    Ensure we are talking about an rdf:Bag

rdf_p_id(LI, _, 'rdf:li') :-
    is_bag_li_predicate(LI),
    !.
rdf_p_id(Resource, DefNS, NSLocal) :-
    rdf_id(Resource, DefNS, NSLocal).

%!  is_bag_li_predicate(+Pred) is semidet.
%
%   True if Pred is _:N, as used  for members of an rdf:Bag, rdf:Seq
%   or rdf:Alt.

is_bag_li_predicate(Pred) :-
    atom_concat('_:', AN, Pred),
    catch(atom_number(AN, N), _, true), integer(N), N >= 0,
    !.


%!  rdf_id(+Resource, +DefNS, -NSLocal)
%
%   Generate a NS:Local name for Resource given the indicated
%   default namespace.  This call is used for elements.

rdf_id(Id, NS, NS:Local) :-
    ns(NS, Full),
    Full \== '',
    atom_concat(Full, Local, Id),
    xml_name(Local),
    !.
rdf_id(Id, _, NS:Local) :-
    ns(NS, Full),
    Full \== '',
    atom_concat(Full, Local, Id),
    xml_name(Local),
    !.
rdf_id(Id, _, Id).


%!  rdf_write_id(+Out, +NSLocal) is det.
%
%   Write an identifier. We cannot use native write on it as both NS
%   and Local can be operators.

rdf_write_id(Out, NS:Local) :-
    !,
    format(Out, '~w:~w', [NS, Local]).
rdf_write_id(Out, Atom) :-
    write(Out, Atom).


%!  rdf_att_id(+URI, +DefNS, -ID)

rdf_att_id(Id, _, NS:Local) :-
    ns(NS, Full),
    Full \== '',
    atom_concat(Full, Local, Id),
    xml_name(Local),
    !.
rdf_att_id(Id, _, Id).


%!  rdf_value(+Resource, -Text, +Encoding)
%
%   According  to  "6.4  RDF  URI  References"  of  the  RDF  Syntax
%   specification, a URI reference is  UNICODE string not containing
%   control sequences, represented as  UTF-8   and  then  as escaped
%   US-ASCII.
%
%   NOTE: the to_be_described/1 trick  ensures   entity  rewrite  in
%   resources that start with 'http://t-d-b.org?'. This   is  a of a
%   hack to save the artchive data   in  the MultimediaN project. We
%   should use a more general mechanism.

rdf_value(V, Text, Encoding) :-
    to_be_described(Prefix),
    atom_concat(Prefix, V1, V),
    ns(NS, Full),
    atom_concat(Full, Local, V1),
    !,
    xml_quote_attribute(Local, QLocal, Encoding),
    atomic_list_concat([Prefix, '&', NS, (';'), QLocal], Text).
rdf_value(V, Text, Encoding) :-
    ns(NS, Full),
    atom_concat(Full, Local, V),
    !,
    xml_quote_attribute(Local, QLocal, Encoding),
    atomic_list_concat(['&', NS, (';'), QLocal], Text).
rdf_value(V, Q, Encoding) :-
    xml_quote_attribute(V, Q, Encoding).

to_be_described('http://t-d-b.org?').


                 /*******************************
                 *             UTIL             *
                 *******************************/

ns(Id, Full) :-
    rdf_db:ns(Id, Full).