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    1/*  Part of SWI-Prolog
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        J.Wielemaker@vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2000-2020, University of Amsterdam
    7                              VU University Amsterdam
    8                              CWI, Amsterdam
    9    All rights reserved.
   10
   11    Redistribution and use in source and binary forms, with or without
   12    modification, are permitted provided that the following conditions
   13    are met:
   14
   15    1. Redistributions of source code must retain the above copyright
   16       notice, this list of conditions and the following disclaimer.
   17
   18    2. Redistributions in binary form must reproduce the above copyright
   19       notice, this list of conditions and the following disclaimer in
   20       the documentation and/or other materials provided with the
   21       distribution.
   22
   23    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   24    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   25    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   26    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
   27    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   28    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   29    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   30    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   31    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   32    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   33    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   34    POSSIBILITY OF SUCH DAMAGE.
   35*/
   36
   37:- module(sgml,
   38          [ load_html/3,                % +Input, -DOM, +Options
   39            load_xml/3,                 % +Input, -DOM, +Options
   40            load_sgml/3,                % +Input, -DOM, +Options
   41
   42            load_sgml_file/2,           % +File, -ListOfContent
   43            load_xml_file/2,            % +File, -ListOfContent
   44            load_html_file/2,           % +File, -Document
   45
   46            load_structure/3,           % +File, -Term, +Options
   47
   48            load_dtd/2,                 % +DTD, +File
   49            load_dtd/3,                 % +DTD, +File, +Options
   50            dtd/2,                      % +Type, -DTD
   51            dtd_property/2,             % +DTD, ?Property
   52
   53            new_dtd/2,                  % +Doctype, -DTD
   54            free_dtd/1,                 % +DTD
   55            open_dtd/3,                 % +DTD, +Options, -Stream
   56
   57            new_sgml_parser/2,          % -Parser, +Options
   58            free_sgml_parser/1,         % +Parser
   59            set_sgml_parser/2,          % +Parser, +Options
   60            get_sgml_parser/2,          % +Parser, +Options
   61            sgml_parse/2,               % +Parser, +Options
   62
   63            sgml_register_catalog_file/2, % +File, +StartOrEnd
   64
   65            xml_quote_attribute/3,      % +In, -Quoted, +Encoding
   66            xml_quote_cdata/3,          % +In, -Quoted, +Encoding
   67            xml_quote_attribute/2,      % +In, -Quoted
   68            xml_quote_cdata/2,          % +In, -Quoted
   69            xml_name/1,                 % +In
   70            xml_name/2,                 % +In, +Encoding
   71
   72            xsd_number_string/2,        % ?Number, ?String
   73            xsd_time_string/3,          % ?Term, ?Type, ?String
   74
   75            xml_basechar/1,             % +Code
   76            xml_ideographic/1,          % +Code
   77            xml_combining_char/1,       % +Code
   78            xml_digit/1,                % +Code
   79            xml_extender/1,             % +Code
   80
   81            iri_xml_namespace/2,        % +IRI, -Namespace
   82            iri_xml_namespace/3,        % +IRI, -Namespace, -LocalName
   83            xml_is_dom/1                % +Term
   84          ]).   85:- autoload(library(error),[instantiation_error/1]).   86:- autoload(library(iostream),[open_any/5,close_any/1]).   87:- autoload(library(lists),[member/2,selectchk/3]).   88:- autoload(library(option),[select_option/3,merge_options/3]).   89
   90:- meta_predicate
   91    load_structure(+, -, :),
   92    load_html(+, -, :),
   93    load_xml(+, -, :),
   94    load_sgml(+, -, :).   95
   96:- predicate_options(load_structure/3, 3,
   97                     [ charpos(integer),
   98                       cdata(oneof([atom,string])),
   99                       defaults(boolean),
  100                       dialect(oneof([html,html4,html5,sgml,xhtml,xhtml5,xml,xmlns])),
  101                       doctype(atom),
  102                       dtd(any),
  103                       encoding(oneof(['iso-8859-1', 'utf-8', 'us-ascii'])),
  104                       entity(atom,atom),
  105                       keep_prefix(boolean),
  106                       file(atom),
  107                       line(integer),
  108                       offset(integer),
  109                       number(oneof([token,integer])),
  110                       qualify_attributes(boolean),
  111                       shorttag(boolean),
  112                       case_sensitive_attributes(boolean),
  113                       case_preserving_attributes(boolean),
  114                       system_entities(boolean),
  115                       max_memory(integer),
  116                       space(oneof([sgml,preserve,default,remove])),
  117                       xmlns(atom),
  118                       xmlns(atom,atom),
  119                       pass_to(sgml_parse/2, 2)
  120                     ]).  121:- predicate_options(load_html/3, 3,
  122                     [ pass_to(load_structure/3, 3)
  123                     ]).  124:- predicate_options(load_xml/3, 3,
  125                     [ pass_to(load_structure/3, 3)
  126                     ]).  127:- predicate_options(load_sgml/3, 3,
  128                     [ pass_to(load_structure/3, 3)
  129                     ]).  130:- predicate_options(load_dtd/3, 3,
  131                     [ dialect(oneof([sgml,xml,xmlns])),
  132                       pass_to(open/4, 4)
  133                     ]).  134:- predicate_options(sgml_parse/2, 2,
  135                     [ call(oneof([begin,end,cdata,pi,decl,error,xmlns,urlns]),
  136                            callable),
  137                       cdata(oneof([atom,string])),
  138                       content_length(integer),
  139                       document(-any),
  140                       max_errors(integer),
  141                       parse(oneof([file,element,content,declaration,input])),
  142                       source(any),
  143                       syntax_errors(oneof([quiet,print,style])),
  144                       xml_no_ns(oneof([error,quiet]))
  145                     ]).  146:- predicate_options(new_sgml_parser/2, 2,
  147                     [ dtd(any)
  148                     ]).

SGML, XML and HTML parser

This library allows you to parse SGML, XML and HTML data into a Prolog data structure. The library defines several families of predicates:

High-level predicates
Most users will only use load_html/3, load_xml/3 or load_sgml/3 to parse arbitrary input into a DOM structure. These predicates all call load_structure/3, which provides more options and may be used for processing non-standard documents.

The DOM structure can be used by library(xpath) to extract information from the document.

The low-level parser
The actual parser is written in C and consists of two parts: one for processing DTD (Document Type Definitions) and one for parsing data. The data can either be parsed to a Prolog (DOM) term or the parser can perform callbacks for the DOM events.
Utility predicates
Finally, this library provides prmitives for classifying characters and strings according to the XML specification such as xml_name/1 to verify whether an atom is a valid XML name (identifier). It also provides primitives to quote attributes and CDATA elements. */
  178:- multifile user:file_search_path/2.  179:- dynamic   user:file_search_path/2.  180
  181user:file_search_path(dtd, '.').
  182user:file_search_path(dtd, swi('library/DTD')).
  183
  184sgml_register_catalog_file(File, Location) :-
  185    prolog_to_os_filename(File, OsFile),
  186    '_sgml_register_catalog_file'(OsFile, Location).
  187
  188:- use_foreign_library(foreign(sgml2pl)).  189
  190register_catalog(Base) :-
  191    absolute_file_name(dtd(Base),
  192                           [ extensions([soc]),
  193                             access(read),
  194                             file_errors(fail)
  195                           ],
  196                           SocFile),
  197    sgml_register_catalog_file(SocFile, end).
  198
  199:- initialization
  200    ignore(register_catalog('HTML4')).  201
  202
  203                 /*******************************
  204                 *         DTD HANDLING         *
  205                 *******************************/
  206
  207/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  208Note that concurrent access to DTD objects  is not allowed, and hence we
  209will allocate and destroy them in each   thread.  Possibibly it would be
  210nicer to find out why  concurrent  access   to  DTD's  is  flawed. It is
  211diagnosed to mess with the entity resolution by Fabien Todescato.
  212- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
  213
  214:- thread_local
  215    current_dtd/2.  216:- volatile
  217    current_dtd/2.  218:- thread_local
  219    registered_cleanup/0.  220:- volatile
  221    registered_cleanup/0.  222
  223:- multifile
  224    dtd_alias/2.  225
  226:- create_prolog_flag(html_dialect, html5, [type(atom)]).  227
  228dtd_alias(html4, 'HTML4').
  229dtd_alias(html5, 'HTML5').
  230dtd_alias(html,  DTD) :-
  231    current_prolog_flag(html_dialect, Dialect),
  232    dtd_alias(Dialect, DTD).
 dtd(+Type, -DTD) is det
DTD is a DTD object created from the file dtd(Type). Loaded DTD objects are cached. Note that DTD objects may not be shared between threads. Therefore, dtd/2 maintains the pool of DTD objects using a thread_local predicate. DTD objects are destroyed if a thread terminates.
Errors
- existence_error(source_sink, dtd(Type))
  244dtd(Type, DTD) :-
  245    current_dtd(Type, DTD),
  246    !.
  247dtd(Type, DTD) :-
  248    new_dtd(Type, DTD),
  249    (   dtd_alias(Type, Base)
  250    ->  true
  251    ;   Base = Type
  252    ),
  253    absolute_file_name(dtd(Base),
  254                       [ extensions([dtd]),
  255                         access(read)
  256                       ], DtdFile),
  257    load_dtd(DTD, DtdFile),
  258    register_cleanup,
  259    asserta(current_dtd(Type, DTD)).
 load_dtd(+DTD, +DtdFile, +Options)
Load DtdFile into a DTD. Defined options are:
dialect(+Dialect)
Dialect to use (xml, xmlns, sgml)
encoding(+Encoding)
Encoding of DTD file
Arguments:
DTD- is a fresh DTD object, normally created using new_dtd/1.
  274load_dtd(DTD, DtdFile) :-
  275    load_dtd(DTD, DtdFile, []).
  276load_dtd(DTD, DtdFile, Options) :-
  277    sgml_open_options(sgml:Options, OpenOptions, sgml:DTDOptions),
  278    setup_call_cleanup(
  279        open_dtd(DTD, DTDOptions, DtdOut),
  280        setup_call_cleanup(
  281            open(DtdFile, read, DtdIn, OpenOptions),
  282            copy_stream_data(DtdIn, DtdOut),
  283            close(DtdIn)),
  284        close(DtdOut)).
  285
  286split_dtd_options([], [], []).
  287split_dtd_options([H|T], [H|TD], S) :-
  288    dtd_option(H),
  289    !,
  290    split_dtd_options(T, TD, S).
  291split_dtd_options([H|T], TD, [H|S]) :-
  292    split_dtd_options(T, TD, S).
  293
  294dtd_option(dialect(_)).
 destroy_dtds
Destroy DTDs cached by this thread as they will become unreachable anyway.
  302destroy_dtds :-
  303    (   current_dtd(_Type, DTD),
  304        free_dtd(DTD),
  305        fail
  306    ;   true
  307    ).
 register_cleanup
Register cleanup of DTDs created for this thread.
  313register_cleanup :-
  314    registered_cleanup,
  315    !.
  316register_cleanup :-
  317    (   current_prolog_flag(threads, true)
  318    ->  prolog_listen(this_thread_exit, destroy_dtds)
  319    ;   true
  320    ),
  321    assert(registered_cleanup).
  322
  323
  324                 /*******************************
  325                 *          EXAMINE DTD         *
  326                 *******************************/
  327
  328prop(doctype(_), _).
  329prop(elements(_), _).
  330prop(entities(_), _).
  331prop(notations(_), _).
  332prop(entity(E, _), DTD) :-
  333    (   nonvar(E)
  334    ->  true
  335    ;   '$dtd_property'(DTD, entities(EL)),
  336        member(E, EL)
  337    ).
  338prop(element(E, _, _), DTD) :-
  339    (   nonvar(E)
  340    ->  true
  341    ;   '$dtd_property'(DTD, elements(EL)),
  342        member(E, EL)
  343    ).
  344prop(attributes(E, _), DTD) :-
  345    (   nonvar(E)
  346    ->  true
  347    ;   '$dtd_property'(DTD, elements(EL)),
  348        member(E, EL)
  349    ).
  350prop(attribute(E, A, _, _), DTD) :-
  351    (   nonvar(E)
  352    ->  true
  353    ;   '$dtd_property'(DTD, elements(EL)),
  354        member(E, EL)
  355    ),
  356    (   nonvar(A)
  357    ->  true
  358    ;   '$dtd_property'(DTD, attributes(E, AL)),
  359        member(A, AL)
  360    ).
  361prop(notation(N, _), DTD) :-
  362    (   nonvar(N)
  363    ->  true
  364    ;   '$dtd_property'(DTD, notations(NL)),
  365        member(N, NL)
  366    ).
  367
  368dtd_property(DTD, Prop) :-
  369    prop(Prop, DTD),
  370    '$dtd_property'(DTD, Prop).
  371
  372
  373                 /*******************************
  374                 *             SGML             *
  375                 *******************************/
 load_structure(+Source, -ListOfContent, :Options) is det
Parse Source and return the resulting structure in ListOfContent. Source is handed to open_any/5, which allows for processing an extensible set of input sources.

A proper XML document contains only a single toplevel element whose name matches the document type. Nevertheless, a list is returned for consistency with the representation of element content.

The encoding(+Encoding) option is treated special for compatibility reasons:

  399load_structure(Spec, DOM, Options) :-
  400    sgml_open_options(Options, OpenOptions, SGMLOptions),
  401    setup_call_cleanup(
  402        open_any(Spec, read, In, Close, OpenOptions),
  403        load_structure_from_stream(In, DOM, SGMLOptions),
  404        close_any(Close)).
  405
  406sgml_open_options(Options, OpenOptions, SGMLOptions) :-
  407    Options = M:Plain,
  408    (   select_option(encoding(Encoding), Plain, NoEnc)
  409    ->  (   sgml_encoding(Encoding)
  410        ->  merge_options(NoEnc, [type(binary)], OpenOptions),
  411            SGMLOptions = Options
  412        ;   OpenOptions = Plain,
  413            SGMLOptions = M:NoEnc
  414        )
  415    ;   merge_options(Plain, [type(binary)], OpenOptions),
  416        SGMLOptions = Options
  417    ).
  418
  419sgml_encoding(Enc) :-
  420    downcase_atom(Enc, Enc1),
  421    sgml_encoding_l(Enc1).
  422
  423sgml_encoding_l('iso-8859-1').
  424sgml_encoding_l('us-ascii').
  425sgml_encoding_l('utf-8').
  426sgml_encoding_l('utf8').
  427sgml_encoding_l('iso_latin_1').
  428sgml_encoding_l('ascii').
  429
  430load_structure_from_stream(In, Term, M:Options) :-
  431    (   select_option(dtd(DTD), Options, Options1)
  432    ->  ExplicitDTD = true
  433    ;   ExplicitDTD = false,
  434        Options1 = Options
  435    ),
  436    move_front(Options1, dialect(_), Options2), % dialect sets defaults
  437    setup_call_cleanup(
  438        new_sgml_parser(Parser,
  439                        [ dtd(DTD)
  440                        ]),
  441        parse(Parser, M:Options2, TermRead, In),
  442        free_sgml_parser(Parser)),
  443    (   ExplicitDTD == true
  444    ->  (   DTD = dtd(_, DocType),
  445            dtd_property(DTD, doctype(DocType))
  446        ->  true
  447        ;   true
  448        )
  449    ;   free_dtd(DTD)
  450    ),
  451    Term = TermRead.
  452
  453move_front(Options0, Opt, Options) :-
  454    selectchk(Opt, Options0, Options1),
  455    !,
  456    Options = [Opt|Options1].
  457move_front(Options, _, Options).
  458
  459
  460parse(Parser, M:Options, Document, In) :-
  461    set_parser_options(Options, Parser, In, Options1),
  462    parser_meta_options(Options1, M, Options2),
  463    set_input_location(Parser, In),
  464    sgml_parse(Parser,
  465               [ document(Document),
  466                 source(In)
  467               | Options2
  468               ]).
  469
  470set_parser_options([], _, _, []).
  471set_parser_options([H|T], Parser, In, Rest) :-
  472    (   set_parser_option(H, Parser, In)
  473    ->  set_parser_options(T, Parser, In, Rest)
  474    ;   Rest = [H|R2],
  475        set_parser_options(T, Parser, In, R2)
  476    ).
  477
  478set_parser_option(Var, _Parser, _In) :-
  479    var(Var),
  480    !,
  481    instantiation_error(Var).
  482set_parser_option(Option, Parser, _) :-
  483    def_entity(Option, Parser),
  484    !.
  485set_parser_option(offset(Offset), _Parser, In) :-
  486    !,
  487    seek(In, Offset, bof, _).
  488set_parser_option(Option, Parser, _In) :-
  489    parser_option(Option),
  490    !,
  491    set_sgml_parser(Parser, Option).
  492set_parser_option(Name=Value, Parser, In) :-
  493    Option =.. [Name,Value],
  494    set_parser_option(Option, Parser, In).
  495
  496
  497parser_option(dialect(_)).
  498parser_option(shorttag(_)).
  499parser_option(case_sensitive_attributes(_)).
  500parser_option(case_preserving_attributes(_)).
  501parser_option(system_entities(_)).
  502parser_option(max_memory(_)).
  503parser_option(file(_)).
  504parser_option(line(_)).
  505parser_option(space(_)).
  506parser_option(number(_)).
  507parser_option(defaults(_)).
  508parser_option(doctype(_)).
  509parser_option(qualify_attributes(_)).
  510parser_option(encoding(_)).
  511parser_option(keep_prefix(_)).
  512
  513
  514def_entity(entity(Name, Value), Parser) :-
  515    get_sgml_parser(Parser, dtd(DTD)),
  516    xml_quote_attribute(Value, QValue),
  517    setup_call_cleanup(open_dtd(DTD, [], Stream),
  518                       format(Stream, '<!ENTITY ~w "~w">~n',
  519                              [Name, QValue]),
  520                       close(Stream)).
  521def_entity(xmlns(URI), Parser) :-
  522    set_sgml_parser(Parser, xmlns(URI)).
  523def_entity(xmlns(NS, URI), Parser) :-
  524    set_sgml_parser(Parser, xmlns(NS, URI)).
 parser_meta_options(+Options0, +Module, -Options)
Qualify meta-calling options to the parser.
  530parser_meta_options([], _, []).
  531parser_meta_options([call(When, Closure)|T0], M, [call(When, M:Closure)|T]) :-
  532    !,
  533    parser_meta_options(T0, M, T).
  534parser_meta_options([H|T0], M, [H|T]) :-
  535    parser_meta_options(T0, M, T).
 set_input_location(+Parser, +In:stream) is det
Set the input location if this was not set explicitly
  542set_input_location(Parser, _In) :-
  543    get_sgml_parser(Parser, file(_)),
  544    !.
  545set_input_location(Parser, In) :-
  546    stream_property(In, file_name(File)),
  547    !,
  548    set_sgml_parser(Parser, file(File)),
  549    stream_property(In, position(Pos)),
  550    set_sgml_parser(Parser, position(Pos)).
  551set_input_location(_, _).
  552
  553                 /*******************************
  554                 *           UTILITIES          *
  555                 *******************************/
 load_sgml_file(+File, -DOM) is det
Load SGML from File and unify the resulting DOM structure with DOM.
deprecated
- New code should use load_sgml/3.
  564load_sgml_file(File, Term) :-
  565    load_sgml(File, Term, []).
 load_xml_file(+File, -DOM) is det
Load XML from File and unify the resulting DOM structure with DOM.
deprecated
- New code should use load_xml/3.
  574load_xml_file(File, Term) :-
  575    load_xml(File, Term, []).
 load_html_file(+File, -DOM) is det
Load HTML from File and unify the resulting DOM structure with DOM.
deprecated
- New code should use load_html/3.
  584load_html_file(File, DOM) :-
  585    load_html(File, DOM, []).
 load_html(+Input, -DOM, +Options) is det
Load HTML text from Input and unify the resulting DOM structure with DOM. Options are passed to load_structure/3, after adding the following default options:
dtd(DTD)
Pass the DTD for HTML as obtained using dtd(html, DTD).
dialect(Dialect)
Current dialect from the Prolog flag html_dialect
max_errors(-1)
syntax_errors(quiet)
Most HTML encountered in the wild contains errors. Even in the context of errors, the resulting DOM term is often a reasonable guess at the intent of the author.

You may also want to use the library(http/http_open) to support loading from HTTP and HTTPS URLs. For example:

:- use_module(library(http/http_open)).
:- use_module(library(sgml)).

load_html_url(URL, DOM) :-
    load_html(URL, DOM, []).
  614load_html(File, Term, M:Options) :-
  615    current_prolog_flag(html_dialect, Dialect),
  616    dtd(Dialect, DTD),
  617    merge_options(Options,
  618                  [ dtd(DTD),
  619                    dialect(Dialect),
  620                    max_errors(-1),
  621                    syntax_errors(quiet)
  622                  ], Options1),
  623    load_structure(File, Term, M:Options1).
 load_xml(+Input, -DOM, +Options) is det
Load XML text from Input and unify the resulting DOM structure with DOM. Options are passed to load_structure/3, after adding the following default options:
  633load_xml(Input, DOM, M:Options) :-
  634    merge_options(Options,
  635                  [ dialect(xml)
  636                  ], Options1),
  637    load_structure(Input, DOM, M:Options1).
 load_sgml(+Input, -DOM, +Options) is det
Load SGML text from Input and unify the resulting DOM structure with DOM. Options are passed to load_structure/3, after adding the following default options:
  647load_sgml(Input, DOM, M:Options) :-
  648    merge_options(Options,
  649                  [ dialect(sgml)
  650                  ], Options1),
  651    load_structure(Input, DOM, M:Options1).
  652
  653
  654
  655                 /*******************************
  656                 *            ENCODING          *
  657                 *******************************/
 xml_quote_attribute(+In, -Quoted) is det
 xml_quote_cdata(+In, -Quoted) is det
Backward compatibility for versions that allow to specify encoding. All characters that cannot fit the encoding are mapped to XML character entities (&#dd;). Using ASCII is the safest value.
  667xml_quote_attribute(In, Quoted) :-
  668    xml_quote_attribute(In, Quoted, ascii).
  669
  670xml_quote_cdata(In, Quoted) :-
  671    xml_quote_cdata(In, Quoted, ascii).
 xml_name(+Atom) is semidet
True if Atom is a valid XML name.
  677xml_name(In) :-
  678    xml_name(In, ascii).
  679
  680
  681                 /*******************************
  682                 *    XML CHARACTER CLASSES     *
  683                 *******************************/
 xml_basechar(+CodeOrChar) is semidet
 xml_ideographic(+CodeOrChar) is semidet
 xml_combining_char(+CodeOrChar) is semidet
 xml_digit(+CodeOrChar) is semidet
 xml_extender(+CodeOrChar) is semidet
XML character classification predicates. Each of these predicates accept both a character (one-character atom) and a code (integer).
See also
- http://www.w3.org/TR/2006/REC-xml-20060816
  698                 /*******************************
  699                 *         TYPE CHECKING        *
  700                 *******************************/
 xml_is_dom(@Term) is semidet
True if term statisfies the structure as returned by load_structure/3 and friends.
  707xml_is_dom(0) :- !, fail.               % catch variables
  708xml_is_dom(List) :-
  709    is_list(List),
  710    !,
  711    xml_is_content_list(List).
  712xml_is_dom(Term) :-
  713    xml_is_element(Term).
  714
  715xml_is_content_list([]).
  716xml_is_content_list([H|T]) :-
  717    xml_is_content(H),
  718    xml_is_content_list(T).
  719
  720xml_is_content(0) :- !, fail.
  721xml_is_content(pi(Pi)) :-
  722    !,
  723    atom(Pi).
  724xml_is_content(CDATA) :-
  725    atom(CDATA),
  726    !.
  727xml_is_content(CDATA) :-
  728    string(CDATA),
  729    !.
  730xml_is_content(Term) :-
  731    xml_is_element(Term).
  732
  733xml_is_element(element(Name, Attributes, Content)) :-
  734    dom_name(Name),
  735    dom_attributes(Attributes),
  736    xml_is_content_list(Content).
  737
  738dom_name(NS:Local) :-
  739    atom(NS),
  740    atom(Local),
  741    !.
  742dom_name(Local) :-
  743    atom(Local).
  744
  745dom_attributes(0) :- !, fail.
  746dom_attributes([]).
  747dom_attributes([H|T]) :-
  748    dom_attribute(H),
  749    dom_attributes(T).
  750
  751dom_attribute(Name=Value) :-
  752    dom_name(Name),
  753    atomic(Value).
  754
  755
  756                 /*******************************
  757                 *            MESSAGES          *
  758                 *******************************/
  759:- multifile
  760    prolog:message/3.  761
  762%       Catch messages.  sgml/4 is generated by the SGML2PL binding.
  763
  764prolog:message(sgml(Parser, File, Line, Message)) -->
  765    { get_sgml_parser(Parser, dialect(Dialect))
  766    },
  767    [ 'SGML2PL(~w): ~w:~w: ~w'-[Dialect, File, Line, Message] ].
  768
  769
  770                 /*******************************
  771                 *         XREF SUPPORT         *
  772                 *******************************/
  773
  774:- multifile
  775    prolog:called_by/2.  776
  777prolog:called_by(sgml_parse(_, Options), Called) :-
  778    findall(Meta, meta_call_term(_, Meta, Options), Called).
  779
  780meta_call_term(T, G+N, Options) :-
  781    T = call(Event, G),
  782    pmember(T, Options),
  783    call_params(Event, Term),
  784    functor(Term, _, N).
  785
  786pmember(X, List) :-                     % member for partial lists
  787    nonvar(List),
  788    List = [H|T],
  789    (   X = H
  790    ;   pmember(X, T)
  791    ).
  792
  793call_params(begin, begin(tag,attributes,parser)).
  794call_params(end,   end(tag,parser)).
  795call_params(cdata, cdata(cdata,parser)).
  796call_params(pi,    pi(cdata,parser)).
  797call_params(decl,  decl(cdata,parser)).
  798call_params(error, error(severity,message,parser)).
  799call_params(xmlns, xmlns(namespace,url,parser)).
  800call_params(urlns, urlns(url,url,parser)).
  801
  802                 /*******************************
  803                 *           SANDBOX            *
  804                 *******************************/
  805
  806:- multifile
  807    sandbox:safe_primitive/1,
  808    sandbox:safe_meta_predicate/1.  809
  810sandbox:safe_meta_predicate(sgml:load_structure/3).
  811sandbox:safe_primitive(sgml:dtd(Dialect, _)) :-
  812    dtd_alias(Dialect, _).
  813sandbox:safe_primitive(sgml:xml_quote_attribute(_,_,_)).
  814sandbox:safe_primitive(sgml:xml_quote_cdata(_,_,_)).
  815sandbox:safe_primitive(sgml:xml_name(_,_)).
  816sandbox:safe_primitive(sgml:xml_basechar(_)).
  817sandbox:safe_primitive(sgml:xml_ideographic(_)).
  818sandbox:safe_primitive(sgml:xml_combining_char(_)).
  819sandbox:safe_primitive(sgml:xml_digit(_)).
  820sandbox:safe_primitive(sgml:xml_extender(_)).
  821sandbox:safe_primitive(sgml:iri_xml_namespace(_,_,_)).
  822sandbox:safe_primitive(sgml:xsd_number_string(_,_)).
  823sandbox:safe_primitive(sgml:xsd_time_string(_,_,_))