/*  Part of SWI-Prolog

    Author:        Jan Wielemaker
    E-mail:        J.Wielemaker@vu.nl
    WWW:           http://www.swi-prolog.org
    Copyright (c)  2009-2023, University of Amsterdam
                              VU University Amsterdam
                              CWI, Amsterdam
                              SWI-Prolog Solutions b.v.
    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;
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*/

:- module('$expand',
          [ expand_term/2,              % +Term0, -Term
            expand_goal/2,              % +Goal0, -Goal
            expand_term/4,              % +Term0, ?Pos0, -Term, -Pos
            expand_goal/4,              % +Goal0, ?Pos0, -Goal, -Pos
            var_property/2,             % +Var, ?Property

            '$including'/0,
            '$expand_closure'/3         % +GoalIn, +Extra, -GoalOut
          ]).

/** <module> Prolog source-code transformation

This module specifies, together with dcg.pl, the transformation of terms
as they are read from a file before they are processed by the compiler.

The toplevel is expand_term/2.  This uses three other translators:

        * Conditional compilation
        * term_expansion/2 rules provided by the user
        * DCG expansion

Note that this ordering implies  that conditional compilation directives
cannot be generated  by  term_expansion/2   rules:  they  must literally
appear in the source-code.

Term-expansion may choose to overrule DCG   expansion.  If the result of
term-expansion is a DCG rule, the rule  is subject to translation into a
predicate.

Next, the result is  passed  to   expand_bodies/2,  which  performs goal
expansion.
*/

:- dynamic
    system:term_expansion/2,
    system:goal_expansion/2,
    user:term_expansion/2,
    user:goal_expansion/2,
    system:term_expansion/4,
    system:goal_expansion/4,
    user:term_expansion/4,
    user:goal_expansion/4.
:- multifile
    system:term_expansion/2,
    system:goal_expansion/2,
    user:term_expansion/2,
    user:goal_expansion/2,
    system:term_expansion/4,
    system:goal_expansion/4,
    user:term_expansion/4,
    user:goal_expansion/4.
:- '$notransact'((system:term_expansion/2,
                  system:goal_expansion/2,
                  user:term_expansion/2,
                  user:goal_expansion/2,
                  system:term_expansion/4,
                  system:goal_expansion/4,
                  user:term_expansion/4,
                  user:goal_expansion/4)).

:- meta_predicate
    expand_terms(4, +, ?, -, -).

%!  expand_term(+Input, -Output) is det.
%!  expand_term(+Input, +Pos0, -Output, -Pos) is det.
%
%   This predicate is used to translate terms  as they are read from
%   a source-file before they are added to the Prolog database.

expand_term(Term0, Term) :-
    expand_term(Term0, _, Term, _).

expand_term(Var, Pos, Expanded, Pos) :-
    var(Var),
    !,
    Expanded = Var.
expand_term(Term, Pos0, [], Pos) :-
    cond_compilation(Term, X),
    X == [],
    !,
    atomic_pos(Pos0, Pos).
expand_term(Term, Pos0, Expanded, Pos) :-
    setup_call_cleanup(
        '$push_input_context'(expand_term),
        expand_term_keep_source_loc(Term, Pos0, Expanded, Pos),
        '$pop_input_context').

expand_term_keep_source_loc(Term, Pos0, Expanded, Pos) :-
    b_setval('$term', Term),
    prepare_directive(Term),
    '$def_modules'([term_expansion/4,term_expansion/2], MList),
    call_term_expansion(MList, Term, Pos0, Term1, Pos1),
    expand_terms(expand_term_2, Term1, Pos1, Expanded, Pos),
    b_setval('$term', []).

%!  prepare_directive(+Directive) is det.
%
%   Try to autoload goals associated with a   directive such that we can
%   allow for term expansion of autoloaded directives such as setting/4.
%   Trying to do so shall raise no errors  nor fail as the directive may
%   be further expanded.

prepare_directive((:- Directive)) :-
    '$current_source_module'(M),
    prepare_directive(Directive, M),
    !.
prepare_directive(_).

prepare_directive(Goal, _) :-
    \+ callable(Goal),
    !.
prepare_directive((A,B), Module) :-
    !,
    prepare_directive(A, Module),
    prepare_directive(B, Module).
prepare_directive(module(_,_), _) :- !.
prepare_directive(Goal, Module) :-
    '$get_predicate_attribute'(Module:Goal, defined, 1),
    !.
prepare_directive(Goal, Module) :-
    \+ current_prolog_flag(autoload, false),
    (   compound(Goal)
    ->  compound_name_arity(Goal, Name, Arity)
    ;   Name = Goal, Arity = 0
    ),
    '$autoload'(Module:Name/Arity),
    !.
prepare_directive(_, _).


call_term_expansion([], Term, Pos, Term, Pos).
call_term_expansion([M-Preds|T], Term0, Pos0, Term, Pos) :-
    current_prolog_flag(sandboxed_load, false),
    !,
    (   '$member'(Pred, Preds),
        (   Pred == term_expansion/2
        ->  M:term_expansion(Term0, Term1),
            Pos1 = Pos0
        ;   M:term_expansion(Term0, Pos0, Term1, Pos1)
        )
    ->  expand_terms(call_term_expansion(T), Term1, Pos1, Term, Pos)
    ;   call_term_expansion(T, Term0, Pos0, Term, Pos)
    ).
call_term_expansion([M-Preds|T], Term0, Pos0, Term, Pos) :-
    (   '$member'(Pred, Preds),
        (   Pred == term_expansion/2
        ->  allowed_expansion(M:term_expansion(Term0, Term1)),
            call(M:term_expansion(Term0, Term1)),
            Pos1 = Pos
        ;   allowed_expansion(M:term_expansion(Term0, Pos0, Term1, Pos1)),
            call(M:term_expansion(Term0, Pos0, Term1, Pos1))
        )
    ->  expand_terms(call_term_expansion(T), Term1, Pos1, Term, Pos)
    ;   call_term_expansion(T, Term0, Pos0, Term, Pos)
    ).

expand_term_2(DCGRule, Pos0, Expanded, Pos) :-
    is_dcg(DCGRule),
    dcg_translate_rule(DCGRule, Pos0, Expanded0, Pos1),
    !,
    expand_bodies(Expanded0, Pos1, Expanded1, Pos),
    non_terminal_decl(Expanded1, Expanded).
expand_term_2(Term0, Pos0, Term, Pos) :-
    nonvar(Term0),
    !,
    expand_bodies(Term0, Pos0, Term, Pos).
expand_term_2(Term, Pos, Term, Pos).

is_dcg(_-->_) => true.
is_dcg(_==>_) => true.
is_dcg(_)     => fail.

non_terminal_decl(Clause, Decl) :-
    \+ current_prolog_flag(xref, true),
    clause_head(Clause, Head),
    '$current_source_module'(M),
    (   '$get_predicate_attribute'(M:Head, non_terminal, NT)
    ->  NT == 0
    ;   true
    ),
    !,
    '$pi_head'(PI, Head),
    Decl = [:-(non_terminal(M:PI)), Clause].
non_terminal_decl(Clause, Clause).

clause_head(Head:-_, Head) :- !.
clause_head(Head, Head).



%!  expand_bodies(+Term, +Pos0, -Out, -Pos) is det.
%
%   Find the body terms in Term and   give them to expand_goal/2 for
%   further processing. Note that  we   maintain  status information
%   about variables. Currently we only  detect whether variables are
%   _fresh_ or not. See var_info/3.

expand_bodies(Terms, Pos0, Out, Pos) :-
    '$def_modules'([goal_expansion/4,goal_expansion/2], MList),
    expand_terms(expand_body(MList), Terms, Pos0, Out, Pos),
    remove_attributes(Out, '$var_info').

expand_body(MList, Clause0, Pos0, Clause, Pos) :-
    clause_head_body(Clause0, Left0, Neck, Body0),
    !,
    clause_head_body(Clause, Left, Neck, Body),
    f2_pos(Pos0, LPos0, BPos0, Pos, LPos, BPos),
    (   head_guard(Left0, Neck, Head0, Guard0)
    ->  f2_pos(LPos0, HPos, GPos0, LPos, HPos, GPos),
        mark_head_variables(Head0),
        expand_goal(Guard0, GPos0, Guard, GPos, MList, Clause0),
        Left = (Head,Guard)
    ;   LPos = LPos0,
        Head0 = Left0,
        Left = Head,
        mark_head_variables(Head0)
    ),
    expand_goal(Body0, BPos0, Body1, BPos, MList, Clause0),
    expand_head_functions(Head0, Head, Body1, Body).
expand_body(MList, (:- Body), Pos0, (:- ExpandedBody), Pos) :-
    !,
    f1_pos(Pos0, BPos0, Pos, BPos),
    expand_goal(Body, BPos0, ExpandedBody, BPos, MList, (:- Body)).

clause_head_body((Head :- Body), Head, :-, Body).
clause_head_body((Head => Body), Head, =>, Body).
clause_head_body(?=>(Head, Body), Head, ?=>, Body).

head_guard(Left, Neck, Head, Guard) :-
    nonvar(Left),
    Left = (Head,Guard),
    (   Neck == (=>)
    ->  true
    ;   Neck == (?=>)
    ).

mark_head_variables(Head) :-
    term_variables(Head, HVars),
    mark_vars_non_fresh(HVars).

expand_head_functions(Head0, Head, Body0, Body) :-
    compound(Head0),
    '$current_source_module'(M),
    replace_functions(Head0, Eval, Head, M),
    Eval \== true,
    !,
    Body = (Eval,Body0).
expand_head_functions(Head, Head, Body, Body).

expand_body(_MList, Head0, Pos, Clause, Pos) :- % TBD: Position handling
    compound(Head0),
    '$current_source_module'(M),
    replace_functions(Head0, Eval, Head, M),
    Eval \== true,
    !,
    Clause = (Head :- Eval).
expand_body(_, Head, Pos, Head, Pos).


%!  expand_terms(:Closure, +In, +Pos0, -Out, -Pos)
%
%   Loop over two constructs that  can   be  added by term-expansion
%   rules in order to run the   next phase: calling term_expansion/2
%   can  return  a  list  and  terms    may   be  preceded  with   a
%   source-location.

expand_terms(_, X, P, X, P) :-
    var(X),
    !.
expand_terms(C, List0, Pos0, List, Pos) :-
    nonvar(List0),
    List0 = [_|_],
    !,
    (   is_list(List0)
    ->  list_pos(Pos0, Elems0, Pos, Elems),
        expand_term_list(C, List0, Elems0, List, Elems)
    ;   '$type_error'(list, List0)
    ).
expand_terms(C, '$source_location'(File, Line):Clause0, Pos0, Clause, Pos) :-
    !,
    expand_terms(C, Clause0, Pos0, Clause1, Pos),
    add_source_location(Clause1, '$source_location'(File, Line), Clause).
expand_terms(C, Term0, Pos0, Term, Pos) :-
    call(C, Term0, Pos0, Term, Pos).

%!  add_source_location(+Term, +SrcLoc, -SrcTerm)
%
%   Re-apply source location after term expansion.  If the result is
%   a list, claim all terms to originate from this location.

add_source_location(Clauses0, SrcLoc, Clauses) :-
    (   is_list(Clauses0)
    ->  add_source_location_list(Clauses0, SrcLoc, Clauses)
    ;   Clauses = SrcLoc:Clauses0
    ).

add_source_location_list([], _, []).
add_source_location_list([Clause|Clauses0], SrcLoc, [SrcLoc:Clause|Clauses]) :-
    add_source_location_list(Clauses0, SrcLoc, Clauses).

%!  expand_term_list(:Expander, +TermList, +Pos, -NewTermList, -PosList)

expand_term_list(_, [], _, [], []) :- !.
expand_term_list(C, [H0|T0], [PH0], Terms, PosL) :-
    !,
    expand_terms(C, H0, PH0, H, PH),
    add_term(H, PH, Terms, TT, PosL, PT),
    expand_term_list(C, T0, [PH0], TT, PT).
expand_term_list(C, [H0|T0], [PH0|PT0], Terms, PosL) :-
    !,
    expand_terms(C, H0, PH0, H, PH),
    add_term(H, PH, Terms, TT, PosL, PT),
    expand_term_list(C, T0, PT0, TT, PT).
expand_term_list(C, [H0|T0], PH0, Terms, PosL) :-
    expected_layout(list, PH0),
    expand_terms(C, H0, PH0, H, PH),
    add_term(H, PH, Terms, TT, PosL, PT),
    expand_term_list(C, T0, [PH0], TT, PT).

%!  add_term(+ExpandOut, ?ExpandPosOut, -Terms, ?TermsT, -PosL, ?PosLT)

add_term(List, Pos, Terms, TermT, PosL, PosT) :-
    nonvar(List), List = [_|_],
    !,
    (   is_list(List)
    ->  append_tp(List, Terms, TermT, Pos, PosL, PosT)
    ;   '$type_error'(list, List)
    ).
add_term(Term, Pos, [Term|Terms], Terms, [Pos|PosT], PosT).

append_tp([], Terms, Terms, _, PosL, PosL).
append_tp([H|T0], [H|T1], Terms, [HP], [HP|TP1], PosL) :-
    !,
    append_tp(T0, T1, Terms, [HP], TP1, PosL).
append_tp([H|T0], [H|T1], Terms, [HP0|TP0], [HP0|TP1], PosL) :-
    !,
    append_tp(T0, T1, Terms, TP0, TP1, PosL).
append_tp([H|T0], [H|T1], Terms, Pos, [Pos|TP1], PosL) :-
    expected_layout(list, Pos),
    append_tp(T0, T1, Terms, [Pos], TP1, PosL).


list_pos(Var, _, _, _) :-
    var(Var),
    !.
list_pos(list_position(F,T,Elems0,none), Elems0,
         list_position(F,T,Elems,none),  Elems) :-
    !.
list_pos(Pos, [Pos], Elems, Elems).


                 /*******************************
                 *      VAR_INFO/3 SUPPORT      *
                 *******************************/

%!  var_intersection(+List1, +List2, -Shared) is det.
%
%   Shared is the ordered intersection of List1 and List2.

var_intersection(List1, List2, Intersection) :-
    sort(List1, Set1),
    sort(List2, Set2),
    ord_intersection(Set1, Set2, Intersection).

%!  ord_intersection(+OSet1, +OSet2, -Int)
%
%   Ordered list intersection.  Copied from the library.

ord_intersection([], _Int, []).
ord_intersection([H1|T1], L2, Int) :-
    isect2(L2, H1, T1, Int).

isect2([], _H1, _T1, []).
isect2([H2|T2], H1, T1, Int) :-
    compare(Order, H1, H2),
    isect3(Order, H1, T1, H2, T2, Int).

isect3(<, _H1, T1,  H2, T2, Int) :-
    isect2(T1, H2, T2, Int).
isect3(=, H1, T1, _H2, T2, [H1|Int]) :-
    ord_intersection(T1, T2, Int).
isect3(>, H1, T1,  _H2, T2, Int) :-
    isect2(T2, H1, T1, Int).

%!  ord_subtract(+Set, +Subtract, -Diff)

ord_subtract([], _Not, []).
ord_subtract(S1, S2, Diff) :-
    S1 == S2,
    !,
    Diff = [].
ord_subtract([H1|T1], L2, Diff) :-
    diff21(L2, H1, T1, Diff).

diff21([], H1, T1, [H1|T1]).
diff21([H2|T2], H1, T1, Diff) :-
    compare(Order, H1, H2),
    diff3(Order, H1, T1, H2, T2, Diff).

diff12([], _H2, _T2, []).
diff12([H1|T1], H2, T2, Diff) :-
    compare(Order, H1, H2),
    diff3(Order, H1, T1, H2, T2, Diff).

diff3(<,  H1, T1,  H2, T2, [H1|Diff]) :-
    diff12(T1, H2, T2, Diff).
diff3(=, _H1, T1, _H2, T2, Diff) :-
    ord_subtract(T1, T2, Diff).
diff3(>,  H1, T1, _H2, T2, Diff) :-
    diff21(T2, H1, T1, Diff).

%!  merge_variable_info(+Saved)
%
%   Merge info from two branches. The  info   in  Saved is the saved
%   info from the  first  branch,  while   the  info  in  the actual
%   variables is the  info  in  the   second  branch.  Only  if both
%   branches claim the variable to  be   fresh,  we  can consider it
%   fresh.

merge_variable_info(State) :-
    catch(merge_variable_info_(State),
          error(uninstantiation_error(Term),_),
          throw(error(goal_expansion_error(bound, Term), _))).

merge_variable_info_([]).
merge_variable_info_([Var=State|States]) :-
    (   get_attr(Var, '$var_info', CurrentState)
    ->  true
    ;   CurrentState = (-)
    ),
    merge_states(Var, State, CurrentState),
    merge_variable_info_(States).

merge_states(_Var, State, State) :- !.
merge_states(_Var, -, _) :- !.
merge_states(Var, State, -) :-
    !,
    put_attr(Var, '$var_info', State).
merge_states(Var, Left, Right) :-
    (   get_dict(fresh, Left, false)
    ->  put_dict(fresh, Right, false)
    ;   get_dict(fresh, Right, false)
    ->  put_dict(fresh, Left, false)
    ),
    !,
    (   Left >:< Right
    ->  put_dict(Left, Right, State),
        put_attr(Var, '$var_info', State)
    ;   print_message(warning,
                      inconsistent_variable_properties(Left, Right)),
        put_dict(Left, Right, State),
        put_attr(Var, '$var_info', State)
    ).


save_variable_info([], []).
save_variable_info([Var|Vars], [Var=State|States]):-
    (   get_attr(Var, '$var_info', State)
    ->  true
    ;   State = (-)
    ),
    save_variable_info(Vars, States).

restore_variable_info(State) :-
    catch(restore_variable_info_(State),
          error(uninstantiation_error(Term),_),
          throw(error(goal_expansion_error(bound, Term), _))).

restore_variable_info_([]).
restore_variable_info_([Var=State|States]) :-
    (   State == (-)
    ->  del_attr(Var, '$var_info')
    ;   put_attr(Var, '$var_info', State)
    ),
    restore_variable_info_(States).

%!  var_property(+Var, ?Property)
%
%   True when Var has a property  Key with Value. Defined properties
%   are:
%
%     - fresh(Fresh)
%     Variable is first introduced in this goal and thus guaranteed
%     to be unbound.  This property is always present.
%     - singleton(Bool)
%     It `true` indicate that the variable appears once in the source.
%     Note this doesn't mean it is a semantic singleton.
%     - name(-Name)
%     True when Name is the name of the variable.

var_property(Var, Property) :-
    prop_var(Property, Var).

prop_var(fresh(Fresh), Var) :-
    (   get_attr(Var, '$var_info', Info),
        get_dict(fresh, Info, Fresh0)
    ->  Fresh = Fresh0
    ;   Fresh = true
    ).
prop_var(singleton(Singleton), Var) :-
    nb_current('$term', Term),
    term_singletons(Term, Singletons),
    (   '$member'(V, Singletons),
        V == Var
    ->  Singleton = true
    ;   Singleton = false
    ).
prop_var(name(Name), Var) :-
    (   nb_current('$variable_names', Bindings),
        '$member'(Name0=Var0, Bindings),
        Var0 == Var
    ->  Name = Name0
    ).


mark_vars_non_fresh([]) :- !.
mark_vars_non_fresh([Var|Vars]) :-
    (   get_attr(Var, '$var_info', Info)
    ->  (   get_dict(fresh, Info, false)
        ->  true
        ;   put_dict(fresh, Info, false, Info1),
            put_attr(Var, '$var_info', Info1)
        )
    ;   put_attr(Var, '$var_info', '$var_info'{fresh:false})
    ),
    mark_vars_non_fresh(Vars).


%!  remove_attributes(+Term, +Attribute) is det.
%
%   Remove all variable attributes Attribute from Term. This is used
%   to make term_expansion end with a  clean term. This is currently
%   _required_ for saving directives  in   QLF  files.  The compiler
%   ignores attributes, but I think  it   is  cleaner to remove them
%   anyway.

remove_attributes(Term, Attr) :-
    term_variables(Term, Vars),
    remove_var_attr(Vars, Attr).

remove_var_attr([], _):- !.
remove_var_attr([Var|Vars], Attr):-
    del_attr(Var, Attr),
    remove_var_attr(Vars, Attr).

%!  '$var_info':attr_unify_hook(_,_) is det.
%
%   Dummy unification hook for attributed variables.  Just succeeds.

'$var_info':attr_unify_hook(_, _).


                 /*******************************
                 *   GOAL_EXPANSION/2 SUPPORT   *
                 *******************************/

%!  expand_goal(+BodyTerm, +Pos0, -Out, -Pos) is det.
%!  expand_goal(+BodyTerm, -Out) is det.
%
%   Perform   macro-expansion   on    body     terms    by   calling
%   goal_expansion/2.

expand_goal(A, B) :-
    expand_goal(A, _, B, _).

expand_goal(A, P0, B, P) :-
    '$def_modules'([goal_expansion/4, goal_expansion/2], MList),
    (   expand_goal(A, P0, B, P, MList, _)
    ->  remove_attributes(B, '$var_info'), A \== B
    ),
    !.
expand_goal(A, P, A, P).

%!  '$expand_closure'(+BodyIn, +ExtraArgs, -BodyOut) is semidet.
%!  '$expand_closure'(+BodyIn, +PIn, +ExtraArgs, -BodyOut, -POut) is semidet.
%
%   Expand a closure using goal expansion  for some extra arguments.
%   Note that the extra argument must remain  at the end. If this is
%   not the case, '$expand_closure'/3,5 fail.

'$expand_closure'(G0, N, G) :-
    '$expand_closure'(G0, _, N, G, _).

'$expand_closure'(G0, P0, N, G, P) :-
    length(Ex, N),
    mark_vars_non_fresh(Ex),
    extend_arg_pos(G0, P0, Ex, G1, P1),
    expand_goal(G1, P1, G2, P2),
    term_variables(G0, VL),
    remove_arg_pos(G2, P2, [], VL, Ex, G, P).


expand_goal(G0, P0, G, P, MList, Term) :-
    '$current_source_module'(M),
    expand_goal(G0, P0, G, P, M, MList, Term, []).

%!  expand_goal(+GoalIn, ?PosIn, -GoalOut, -PosOut,
%!              +Module, -ModuleList, +Term, +Done) is det.
%
%   @arg Module is the current module to consider
%   @arg ModuleList are the other expansion modules
%   @arg Term is the overall term that is being translated
%   @arg Done is a list of terms that have already been expanded

% (*)   This is needed because call_goal_expansion may introduce extra
%       context variables.  Consider the code below, where the variable
%       E is introduced.  Is there a better representation for the
%       context?
%
%         ==
%         goal_expansion(catch_and_print(Goal), catch(Goal, E, print(E))).
%
%         test :-
%               catch_and_print(true).
%         ==

expand_goal(G, P, G, P, _, _, _, _) :-
    var(G),
    !.
expand_goal(M:G, P, M:G, P, _M, _MList, _Term, _) :-
    var(M), var(G),
    !.
expand_goal(M:G, P0, M:EG, P, _M, _MList, Term, Done) :-
    atom(M),
    !,
    f2_pos(P0, PA, PB0, P, PA, PB),
    '$def_modules'(M:[goal_expansion/4,goal_expansion/2], MList),
    setup_call_cleanup(
        '$set_source_module'(Old, M),
        '$expand':expand_goal(G, PB0, EG, PB, M, MList, Term, Done),
        '$set_source_module'(Old)).
expand_goal(G0, P0, G, P, M, MList, Term, Done) :-
    (   already_expanded(G0, Done, Done1)
    ->  expand_control(G0, P0, G, P, M, MList, Term, Done1)
    ;   call_goal_expansion(MList, G0, P0, G1, P1)
    ->  expand_goal(G1, P1, G, P, M, MList, Term/G1, [G0|Done])      % (*)
    ;   expand_control(G0, P0, G, P, M, MList, Term, Done)
    ).

expand_control((A,B), P0, Conj, P, M, MList, Term, Done) :-
    !,
    f2_pos(P0, PA0, PB0, P1, PA, PB),
    expand_goal(A, PA0, EA, PA, M, MList, Term, Done),
    expand_goal(B, PB0, EB, PB, M, MList, Term, Done),
    simplify((EA,EB), P1, Conj, P).
expand_control((A;B), P0, Or, P, M, MList, Term, Done) :-
    !,
    f2_pos(P0, PA0, PB0, P1, PA1, PB),
    term_variables(A, AVars),
    term_variables(B, BVars),
    var_intersection(AVars, BVars, SharedVars),
    save_variable_info(SharedVars, SavedState),
    expand_goal(A, PA0, EA, PA, M, MList, Term, Done),
    save_variable_info(SharedVars, SavedState2),
    restore_variable_info(SavedState),
    expand_goal(B, PB0, EB, PB, M, MList, Term, Done),
    merge_variable_info(SavedState2),
    fixup_or_lhs(A, EA, PA, EA1, PA1),
    simplify((EA1;EB), P1, Or, P).
expand_control((A->B), P0, Goal, P, M, MList, Term, Done) :-
    !,
    f2_pos(P0, PA0, PB0, P1, PA, PB),
    expand_goal(A, PA0, EA, PA, M, MList, Term, Done),
    expand_goal(B, PB0, EB, PB, M, MList, Term, Done),
    simplify((EA->EB), P1, Goal, P).
expand_control((A*->B), P0, Goal, P, M, MList, Term, Done) :-
    !,
    f2_pos(P0, PA0, PB0, P1, PA, PB),
    expand_goal(A, PA0, EA, PA, M, MList, Term, Done),
    expand_goal(B, PB0, EB, PB, M, MList, Term, Done),
    simplify((EA*->EB), P1, Goal, P).
expand_control((\+A), P0, Goal, P, M, MList, Term, Done) :-
    !,
    f1_pos(P0, PA0, P1, PA),
    term_variables(A, AVars),
    save_variable_info(AVars, SavedState),
    expand_goal(A, PA0, EA, PA, M, MList, Term, Done),
    restore_variable_info(SavedState),
    simplify(\+(EA), P1, Goal, P).
expand_control(call(A), P0, call(EA), P, M, MList, Term, Done) :-
    !,
    f1_pos(P0, PA0, P, PA),
    expand_goal(A, PA0, EA, PA, M, MList, Term, Done).
expand_control($(A), P0, $(EA), P, M, MList, Term, Done) :-
    !,
    f1_pos(P0, PA0, P, PA),
    expand_goal(A, PA0, EA, PA, M, MList, Term, Done).
expand_control(G0, P0, G, P, M, MList, Term, Done) :-
    is_meta_call(G0, M, Head),
    !,
    term_variables(G0, Vars),
    mark_vars_non_fresh(Vars),
    expand_meta(Head, G0, P0, G, P, M, MList, Term, Done).
expand_control(G0, P0, G, P, M, MList, Term, _Done) :-
    term_variables(G0, Vars),
    mark_vars_non_fresh(Vars),
    expand_functions(G0, P0, G, P, M, MList, Term).

%!  already_expanded(+Goal, +Done, -RestDone) is semidet.

already_expanded(Goal, Done, Done1) :-
    '$select'(G, Done, Done1),
    G == Goal,
    !.

%!  fixup_or_lhs(+OldLeft, -ExpandedLeft, +ExpPos, -Fixed, -FixedPos) is det.
%
%   The semantics of (A;B) is different if  A is (If->Then). We need
%   to keep the same semantics if -> is introduced or removed by the
%   expansion. If -> is introduced, we make sure that the whole
%   thing remains a disjunction by creating ((EA,true);B)

fixup_or_lhs(Old, New, PNew, Fix, PFixed) :-
    nonvar(Old),
    nonvar(New),
    (   Old = (_ -> _)
    ->  New \= (_ -> _),
        Fix = (New -> true)
    ;   New = (_ -> _),
        Fix = (New, true)
    ),
    !,
    lhs_pos(PNew, PFixed).
fixup_or_lhs(_Old, New, P, New, P).

lhs_pos(P0, _) :-
    var(P0),
    !.
lhs_pos(P0, term_position(F,T,T,T,[P0,T-T])) :-
    arg(1, P0, F),
    arg(2, P0, T).


%!  is_meta_call(+G0, +M, -Head) is semidet.
%
%   True if M:G0 resolves to a real meta-goal as specified by Head.

is_meta_call(G0, M, Head) :-
    compound(G0),
    default_module(M, M2),
    '$c_current_predicate'(_, M2:G0),
    !,
    '$get_predicate_attribute'(M2:G0, meta_predicate, Head),
    has_meta_arg(Head).


%!  expand_meta(+MetaSpec, +G0, ?P0, -G, -P, +M, +Mlist, +Term, +Done)

expand_meta(Spec, G0, P0, G, P, M, MList, Term, Done) :-
    functor(Spec, _, Arity),
    functor(G0, Name, Arity),
    functor(G1, Name, Arity),
    f_pos(P0, ArgPos0, G1P, ArgPos),
    expand_meta(1, Arity, Spec,
                G0, ArgPos0, Eval, EvalPos,
                G1,  ArgPos,
                M, MList, Term, Done),
    conj(Eval, EvalPos, G1, G1P, G, P).

expand_meta(I, Arity, Spec, G0, ArgPos0, Eval, EvalPos, G, [P|PT],
            M, MList, Term, Done) :-
    I =< Arity,
    !,
    arg_pos(ArgPos0, P0, PT0),
    arg(I, Spec, Meta),
    arg(I, G0, A0),
    arg(I, G, A),
    expand_meta_arg(Meta, A0, P0, EvalA, EPA, A, P, M, MList, Term, Done),
    I2 is I + 1,
    expand_meta(I2, Arity, Spec, G0, PT0, EvalB,EPB, G, PT, M, MList, Term, Done),
    conj(EvalA, EPA, EvalB, EPB, Eval, EvalPos).
expand_meta(_, _, _, _, _, true, _, _, [], _, _, _, _).

arg_pos(List, _, _) :- var(List), !.    % no position info
arg_pos([H|T], H, T) :- !.              % argument list
arg_pos([], _, []).                     % new has more

mapex([], _).
mapex([E|L], E) :- mapex(L, E).

%!  extended_pos(+Pos0, +N, -Pos) is det.
%!  extended_pos(-Pos0, +N, +Pos) is det.
%
%   Pos is the result of adding N extra positions to Pos0.

extended_pos(Var, _, Var) :-
    var(Var),
    !.
extended_pos(parentheses_term_position(O,C,Pos0),
             N,
             parentheses_term_position(O,C,Pos)) :-
    !,
    extended_pos(Pos0, N, Pos).
extended_pos(term_position(F,T,FF,FT,Args),
             _,
             term_position(F,T,FF,FT,Args)) :-
    var(Args),
    !.
extended_pos(term_position(F,T,FF,FT,Args0),
             N,
             term_position(F,T,FF,FT,Args)) :-
    length(Ex, N),
    mapex(Ex, T-T),
    '$append'(Args0, Ex, Args),
    !.
extended_pos(F-T,
             N,
             term_position(F,T,F,T,Ex)) :-
    !,
    length(Ex, N),
    mapex(Ex, T-T).
extended_pos(Pos, N, Pos) :-
    '$print_message'(warning, extended_pos(Pos, N)).

%!  expand_meta_arg(+MetaSpec, +Arg0, +ArgPos0, -Eval, -EvalPos,
%!                  -Arg, -ArgPos, +ModuleList, +Term, +Done) is det.
%
%   Goal expansion for a meta-argument.
%
%   @arg    Eval is always `true`.  Future versions should allow for
%           functions on such positions.  This requires proper
%           position management for function expansion.

expand_meta_arg(0, A0, PA0, true, _, A, PA, M, MList, Term, Done) :-
    !,
    expand_goal(A0, PA0, A1, PA, M, MList, Term, Done),
    compile_meta_call(A1, A, M, Term).
expand_meta_arg(N, A0, P0, true, _, A, P, M, MList, Term, Done) :-
    integer(N), callable(A0),
    replace_functions(A0, true, _, M),
    !,
    length(Ex, N),
    mark_vars_non_fresh(Ex),
    extend_arg_pos(A0, P0, Ex, A1, PA1),
    expand_goal(A1, PA1, A2, PA2, M, MList, Term, Done),
    compile_meta_call(A2, A3, M, Term),
    term_variables(A0, VL),
    remove_arg_pos(A3, PA2, M, VL, Ex, A, P).
expand_meta_arg(^, A0, PA0, true, _, A, PA, M, MList, Term, Done) :-
    !,
    expand_setof_goal(A0, PA0, A, PA, M, MList, Term, Done).
expand_meta_arg(S, A0, PA0, Eval, EPA, A, PA, M, _MList, _Term, _Done) :-
    replace_functions(A0, PA0, Eval, EPA, A, PA, M),
    (   Eval == true
    ->  true
    ;   same_functor(A0, A)
    ->  true
    ;   meta_arg(S)
    ->  throw(error(context_error(function, meta_arg(S)), _))
    ;   true
    ).

same_functor(T1, T2) :-
    compound(T1),
    !,
    compound(T2),
    compound_name_arity(T1, N, A),
    compound_name_arity(T2, N, A).
same_functor(T1, T2) :-
    atom(T1),
    T1 == T2.

variant_sha1_nat(Term, Hash) :-
    copy_term_nat(Term, TNat),
    variant_sha1(TNat, Hash).

wrap_meta_arguments(A0, M, VL, Ex, A) :-
    '$append'(VL, Ex, AV),
    variant_sha1_nat(A0+AV, Hash),
    atom_concat('__aux_wrapper_', Hash, AuxName),
    H =.. [AuxName|AV],
    compile_auxiliary_clause(M, (H :- A0)),
    A =.. [AuxName|VL].

%!  extend_arg_pos(+A0, +P0, +Ex, -A, -P) is det.
%
%   Adds extra arguments Ex to A0, and  extra subterm positions to P
%   for such arguments.

extend_arg_pos(A, P, _, A, P) :-
    var(A),
    !.
extend_arg_pos(M:A0, P0, Ex, M:A, P) :-
    !,
    f2_pos(P0, PM, PA0, P, PM, PA),
    extend_arg_pos(A0, PA0, Ex, A, PA).
extend_arg_pos(A0, P0, Ex, A, P) :-
    callable(A0),
    !,
    extend_term(A0, Ex, A),
    length(Ex, N),
    extended_pos(P0, N, P).
extend_arg_pos(A, P, _, A, P).

extend_term(Atom, Extra, Term) :-
    atom(Atom),
    !,
    Term =.. [Atom|Extra].
extend_term(Term0, Extra, Term) :-
    compound_name_arguments(Term0, Name, Args0),
    '$append'(Args0, Extra, Args),
    compound_name_arguments(Term, Name, Args).

%!  remove_arg_pos(+A0, +P0, +M, +Ex, +VL, -A, -P) is det.
%
%   Removes the Ex arguments  from  A0   and  the  respective  extra
%   positions from P0. Note that  if  they   are  not  at the end, a
%   wrapper with the elements of VL as arguments is generated to put
%   them in order.
%
%   @see wrap_meta_arguments/5

remove_arg_pos(A, P, _, _, _, A, P) :-
    var(A),
    !.
remove_arg_pos(M:A0, P0, _, VL, Ex, M:A, P) :-
    !,
    f2_pos(P, PM, PA0, P0, PM, PA),
    remove_arg_pos(A0, PA, M, VL, Ex, A, PA0).
remove_arg_pos(A0, P0, M, VL, Ex0, A, P) :-
    callable(A0),
    !,
    length(Ex0, N),
    (   A0 =.. [F|Args],
        length(Ex, N),
        '$append'(Args0, Ex, Args),
        Ex==Ex0
    ->  extended_pos(P, N, P0),
        A =.. [F|Args0]
    ;   M \== [],
        wrap_meta_arguments(A0, M, VL, Ex0, A),
        wrap_meta_pos(P0, P)
    ).
remove_arg_pos(A, P, _, _, _, A, P).

wrap_meta_pos(P0, P) :-
    (   nonvar(P0)
    ->  P = term_position(F,T,_,_,_),
        atomic_pos(P0, F-T)
    ;   true
    ).

has_meta_arg(Head) :-
    arg(_, Head, Arg),
    direct_call_meta_arg(Arg),
    !.

direct_call_meta_arg(I) :- integer(I).
direct_call_meta_arg(^).

meta_arg(:).
meta_arg(//).
meta_arg(I) :- integer(I).

expand_setof_goal(Var, Pos, Var, Pos, _, _, _, _) :-
    var(Var),
    !.
expand_setof_goal(V^G, P0, V^EG, P, M, MList, Term, Done) :-
    !,
    f2_pos(P0, PA0, PB, P, PA, PB),
    expand_setof_goal(G, PA0, EG, PA, M, MList, Term, Done).
expand_setof_goal(M0:G, P0, M0:EG, P, M, MList, Term, Done) :-
    !,
    f2_pos(P0, PA0, PB, P, PA, PB),
    expand_setof_goal(G, PA0, EG, PA, M, MList, Term, Done).
expand_setof_goal(G, P0, EG, P, M, MList, Term, Done) :-
    !,
    expand_goal(G, P0, EG0, P, M, MList, Term, Done),
    compile_meta_call(EG0, EG1, M, Term),
    (   extend_existential(G, EG1, V)
    ->  EG = V^EG1
    ;   EG = EG1
    ).

%!  extend_existential(+G0, +G1, -V) is semidet.
%
%   Extend  the  variable  template  to    compensate  for  intermediate
%   variables introduced during goal expansion   (notably for functional
%   notation).

extend_existential(G0, G1, V) :-
    term_variables(G0, GV0), sort(GV0, SV0),
    term_variables(G1, GV1), sort(GV1, SV1),
    ord_subtract(SV1, SV0, New),
    New \== [],
    V =.. [v|New].

%!  call_goal_expansion(+ExpandModules,
%!                      +Goal0, ?Pos0, -Goal, -Pos, +Done) is semidet.
%
%   Succeeds  if  the   context   has    a   module   that   defines
%   goal_expansion/2 this rule succeeds and  Goal   is  not equal to
%   Goal0. Note that the translator is   called  recursively until a
%   fixed-point is reached.

call_goal_expansion(MList, G0, P0, G, P) :-
    current_prolog_flag(sandboxed_load, false),
    !,
    (   '$member'(M-Preds, MList),
        '$member'(Pred, Preds),
        (   Pred == goal_expansion/4
        ->  M:goal_expansion(G0, P0, G, P)
        ;   M:goal_expansion(G0, G),
            P = P0
        ),
        G0 \== G
    ->  true
    ).
call_goal_expansion(MList, G0, P0, G, P) :-
    (   '$member'(M-Preds, MList),
        '$member'(Pred, Preds),
        (   Pred == goal_expansion/4
        ->  Expand = M:goal_expansion(G0, P0, G, P)
        ;   Expand = M:goal_expansion(G0, G)
        ),
        allowed_expansion(Expand),
        call(Expand),
        G0 \== G
    ->  true
    ).

%!  allowed_expansion(:Goal) is semidet.
%
%   Calls prolog:sandbox_allowed_expansion(:Goal) prior   to calling
%   Goal for the purpose of term or   goal  expansion. This hook can
%   prevent the expansion to take place by raising an exception.
%
%   @throws exceptions from prolog:sandbox_allowed_expansion/1.

:- multifile
    prolog:sandbox_allowed_expansion/1.

allowed_expansion(QGoal) :-
    strip_module(QGoal, M, Goal),
    E = error(Formal,_),
    catch(prolog:sandbox_allowed_expansion(M:Goal), E, true),
    (   var(Formal)
    ->  fail
    ;   !,
        print_message(error, E),
        fail
    ).
allowed_expansion(_).


                 /*******************************
                 *      FUNCTIONAL NOTATION     *
                 *******************************/

%!  expand_functions(+G0, +P0, -G, -P, +M, +MList, +Term) is det.
%
%   Expand functional notation and arithmetic functions.
%
%   @arg MList is the list of modules defining goal_expansion/2 in
%   the expansion context.

expand_functions(G0, P0, G, P, M, MList, Term) :-
    expand_functional_notation(G0, P0, G1, P1, M, MList, Term),
    (   expand_arithmetic(G1, P1, G, P, Term)
    ->  true
    ;   G = G1,
        P = P1
    ).

%!  expand_functional_notation(+G0, +P0, -G, -P, +M, +MList, +Term) is det.
%
%   @tbd: position logic
%   @tbd: make functions module-local

expand_functional_notation(G0, P0, G, P, M, _MList, _Term) :-
    contains_functions(G0),
    replace_functions(G0, P0, Eval, EvalPos, G1, G1Pos, M),
    Eval \== true,
    !,
    wrap_var(G1, G1Pos, G2, G2Pos),
    conj(Eval, EvalPos, G2, G2Pos, G, P).
expand_functional_notation(G, P, G, P, _, _, _).

wrap_var(G, P, G, P) :-
    nonvar(G),
    !.
wrap_var(G, P0, call(G), P) :-
    (   nonvar(P0)
    ->  P = term_position(F,T,F,T,[P0]),
        atomic_pos(P0, F-T)
    ;   true
    ).

%!  contains_functions(@Term) is semidet.
%
%   True when Term contains a function reference.

contains_functions(Term) :-
    \+ \+ ( '$factorize_term'(Term, Skeleton, Assignments),
            (   contains_functions2(Skeleton)
            ;   contains_functions2(Assignments)
            )).

contains_functions2(Term) :-
    compound(Term),
    (   function(Term, _)
    ->  true
    ;   arg(_, Term, Arg),
        contains_functions2(Arg)
    ->  true
    ).

%!  replace_functions(+GoalIn, +PosIn,
%!                    -Eval, -EvalPos,
%!                    -GoalOut, -PosOut,
%!                    +ContextTerm) is det.
%
%   @tbd    Proper propagation of list, dict and brace term positions.

:- public
    replace_functions/4.            % used in dicts.pl

replace_functions(GoalIn, Eval, GoalOut, Context) :-
    replace_functions(GoalIn, _, Eval, _, GoalOut, _, Context).

replace_functions(Var, Pos, true, _, Var, Pos, _Ctx) :-
    var(Var),
    !.
replace_functions(F, FPos, Eval, EvalPos, Var, VarPos, Ctx) :-
    function(F, Ctx),
    !,
    compound_name_arity(F, Name, Arity),
    PredArity is Arity+1,
    compound_name_arity(G, Name, PredArity),
    arg(PredArity, G, Var),
    extend_1_pos(FPos, FArgPos, GPos, GArgPos, VarPos),
    map_functions(0, Arity, F, FArgPos, G, GArgPos, Eval0, EP0, Ctx),
    conj(Eval0, EP0, G, GPos, Eval, EvalPos).
replace_functions(Term0, Term0Pos, Eval, EvalPos, Term, TermPos, Ctx) :-
    compound(Term0),
    !,
    compound_name_arity(Term0, Name, Arity),
    compound_name_arity(Term, Name, Arity),
    f_pos(Term0Pos, Args0Pos, TermPos, ArgsPos),
    map_functions(0, Arity,
                  Term0, Args0Pos, Term, ArgsPos, Eval, EvalPos, Ctx).
replace_functions(Term, Pos, true, _, Term, Pos, _).


%!  map_functions(+Arg, +Arity,
%!                +TermIn, +ArgInPos, -Term, -ArgPos, -Eval, -EvalPos,
%!                +Context)

map_functions(Arity, Arity, _, LPos0, _, LPos, true, _, _) :-
    !,
    pos_nil(LPos0, LPos).
map_functions(I0, Arity, Term0, LPos0, Term, LPos, Eval, EP, Ctx) :-
    pos_list(LPos0, AP0, APT0, LPos, AP, APT),
    I is I0+1,
    arg(I, Term0, Arg0),
    arg(I, Term, Arg),
    replace_functions(Arg0, AP0, Eval0, EP0, Arg, AP, Ctx),
    map_functions(I, Arity, Term0, APT0, Term, APT, Eval1, EP1, Ctx),
    conj(Eval0, EP0, Eval1, EP1, Eval, EP).

%!  conj(+G1, +P1, +G2, +P2, -G, -P)

conj(true, _, X, P, X, P) :- !.
conj(X, P, true, _, X, P) :- !.
conj(X, PX, Y, PY, (X,Y), _) :-
    var(PX), var(PY),
    !.
conj(X, PX, Y, PY, (X,Y), P) :-
    P = term_position(F,T,FF,FT,[PX,PY]),
    atomic_pos(PX, F-FF),
    atomic_pos(PY, FT-T).

%!  function(?Term, +Context)
%
%   True if function expansion needs to be applied for the given
%   term.

:- multifile
    function/2.

function(.(_,_), _) :- \+ functor([_|_], ., _).


                 /*******************************
                 *          ARITHMETIC          *
                 *******************************/

%!  expand_arithmetic(+G0, +P0, -G, -P, +Term) is semidet.
%
%   Expand arithmetic expressions  in  is/2,   (>)/2,  etc.  This is
%   currently a dummy.  The  idea  is   to  call  rules  similar  to
%   goal_expansion/2,4  that  allow  for   rewriting  an  arithmetic
%   expression. The system rules will perform evaluation of constant
%   expressions.

expand_arithmetic(_G0, _P0, _G, _P, _Term) :- fail.


                 /*******************************
                 *        POSITION LOGIC        *
                 *******************************/

%!  f2_pos(?TermPos0, ?PosArg10, ?PosArg20,
%!         ?TermPos,  ?PosArg1,  ?PosArg2) is det.
%!  f1_pos(?TermPos0, ?PosArg10, ?TermPos,  ?PosArg1) is det.
%!  f_pos(?TermPos0, ?PosArgs0, ?TermPos,  ?PosArgs) is det.
%!  atomic_pos(?TermPos0, -AtomicPos) is det.
%
%   Position progapation routines.

f2_pos(Var, _, _, _, _, _) :-
    var(Var),
    !.
f2_pos(term_position(F,T,FF,FT,[A10,A20]), A10, A20,
       term_position(F,T,FF,FT,[A1, A2 ]), A1,  A2) :- !.
f2_pos(parentheses_term_position(O,C,Pos0), A10, A20,
       parentheses_term_position(O,C,Pos),  A1,  A2) :-
    !,
    f2_pos(Pos0, A10, A20, Pos, A1, A2).
f2_pos(Pos, _, _, _, _, _) :-
    expected_layout(f2, Pos).

f1_pos(Var, _, _, _) :-
    var(Var),
    !.
f1_pos(term_position(F,T,FF,FT,[A10]), A10,
       term_position(F,T,FF,FT,[A1 ]),  A1) :- !.
f1_pos(parentheses_term_position(O,C,Pos0), A10,
       parentheses_term_position(O,C,Pos),  A1) :-
    !,
    f1_pos(Pos0, A10, Pos, A1).
f1_pos(Pos, _, _, _) :-
    expected_layout(f1, Pos).

f_pos(Var, _, _, _) :-
    var(Var),
    !.
f_pos(term_position(F,T,FF,FT,ArgPos0), ArgPos0,
      term_position(F,T,FF,FT,ArgPos),  ArgPos) :- !.
f_pos(parentheses_term_position(O,C,Pos0), A10,
      parentheses_term_position(O,C,Pos),  A1) :-
    !,
    f_pos(Pos0, A10, Pos, A1).
f_pos(Pos, _, _, _) :-
    expected_layout(compound, Pos).

atomic_pos(Pos, _) :-
    var(Pos),
    !.
atomic_pos(Pos, F-T) :-
    arg(1, Pos, F),
    arg(2, Pos, T).

%!  pos_nil(+Nil, -Nil) is det.
%!  pos_list(+List0, -H0, -T0, -List, -H, -T) is det.
%
%   Position propagation for lists.

pos_nil(Var, _) :- var(Var), !.
pos_nil([], []) :- !.
pos_nil(Pos, _) :-
    expected_layout(nil, Pos).

pos_list(Var, _, _, _, _, _) :- var(Var), !.
pos_list([H0|T0], H0, T0, [H|T], H, T) :- !.
pos_list(Pos, _, _, _, _, _) :-
    expected_layout(list, Pos).

%!  extend_1_pos(+FunctionPos, -FArgPos, -EvalPos, -EArgPos, -VarPos)
%
%   Deal with extending a function to include the return value.

extend_1_pos(Pos, _, _, _, _) :-
    var(Pos),
    !.
extend_1_pos(term_position(F,T,FF,FT,FArgPos), FArgPos,
             term_position(F,T,FF,FT,GArgPos), GArgPos0,
             FT-FT1) :-
    integer(FT),
    !,
    FT1 is FT+1,
    '$same_length'(FArgPos, GArgPos0),
    '$append'(GArgPos0, [FT-FT1], GArgPos).
extend_1_pos(F-T, [],
             term_position(F,T,F,T,[T-T1]), [],
             T-T1) :-
    integer(T),
    !,
    T1 is T+1.
extend_1_pos(Pos, _, _, _, _) :-
    expected_layout(callable, Pos).

'$same_length'(List, List) :-
    var(List),
    !.
'$same_length'([], []).
'$same_length'([_|T0], [_|T]) :-
    '$same_length'(T0, T).


%!  expected_layout(+Expected, +Found)
%
%   Print a message  if  the  layout   term  does  not  satisfy  our
%   expectations.  This  means  that   the  transformation  requires
%   support from term_expansion/4 and/or goal_expansion/4 to achieve
%   proper source location information.

:- create_prolog_flag(debug_term_position, false, []).

expected_layout(Expected, Pos) :-
    current_prolog_flag(debug_term_position, true),
    !,
    '$print_message'(warning, expected_layout(Expected, Pos)).
expected_layout(_, _).


                 /*******************************
                 *    SIMPLIFICATION ROUTINES   *
                 *******************************/

%!  simplify(+ControlIn, +Pos0, -ControlOut, -Pos) is det.
%
%   Simplify control structures
%
%   @tbd    Much more analysis
%   @tbd    Turn this into a separate module

simplify(Control, P, Control, P) :-
    current_prolog_flag(optimise, false),
    !.
simplify(Control, P0, Simple, P) :-
    simple(Control, P0, Simple, P),
    !.
simplify(Control, P, Control, P).

%!  simple(+Goal, +GoalPos, -Simple, -SimplePos)
%
%   Simplify a control structure.  Note  that   we  do  not simplify
%   (A;fail). Logically, this is the  same  as   `A`  if  `A` is not
%   `_->_` or `_*->_`, but  the  choice   point  may  be  created on
%   purpose.

simple((X,Y), P0, Conj, P) :-
    (   true(X)
    ->  Conj = Y,
        f2_pos(P0, _, P, _, _, _)
    ;   false(X)
    ->  Conj = fail,
        f2_pos(P0, P1, _, _, _, _),
        atomic_pos(P1, P)
    ;   true(Y)
    ->  Conj = X,
        f2_pos(P0, P, _, _, _, _)
    ).
simple((I->T;E), P0, ITE, P) :-         % unification with _->_ is fine
    (   true(I)                     % because nothing happens if I and T
    ->  ITE = T,                    % are unbound.
        f2_pos(P0, P1, _, _, _, _),
        f2_pos(P1, _, P, _, _, _)
    ;   false(I)
    ->  ITE = E,
        f2_pos(P0, _, P, _, _, _)
    ).
simple((X;Y), P0, Or, P) :-
    false(X),
    Or = Y,
    f2_pos(P0, _, P, _, _, _).

true(X) :-
    nonvar(X),
    eval_true(X).

false(X) :-
    nonvar(X),
    eval_false(X).


%!  eval_true(+Goal) is semidet.
%!  eval_false(+Goal) is semidet.

eval_true(true).
eval_true(otherwise).

eval_false(fail).
eval_false(false).


                 /*******************************
                 *         META CALLING         *
                 *******************************/

:- create_prolog_flag(compile_meta_arguments, false, [type(atom)]).

%!  compile_meta_call(+CallIn, -CallOut, +Module, +Term) is det.
%
%   Compile (complex) meta-calls into a clause.

compile_meta_call(CallIn, CallIn, _, Term) :-
    var(Term),
    !.                   % explicit call; no context
compile_meta_call(CallIn, CallIn, _, _) :-
    var(CallIn),
    !.
compile_meta_call(CallIn, CallIn, _, _) :-
    (   current_prolog_flag(compile_meta_arguments, false)
    ;   current_prolog_flag(xref, true)
    ),
    !.
compile_meta_call(CallIn, CallIn, _, _) :-
    strip_module(CallIn, _, Call),
    (   is_aux_meta(Call)
    ;   \+ control(Call),
        (   '$c_current_predicate'(_, system:Call),
            \+ current_prolog_flag(compile_meta_arguments, always)
        ;   current_prolog_flag(compile_meta_arguments, control)
        )
    ),
    !.
compile_meta_call(M:CallIn, CallOut, _, Term) :-
    !,
    (   atom(M), callable(CallIn)
    ->  compile_meta_call(CallIn, CallOut, M, Term)
    ;   CallOut = M:CallIn
    ).
compile_meta_call(CallIn, CallOut, Module, Term) :-
    compile_meta(CallIn, CallOut, Module, Term, Clause),
    compile_auxiliary_clause(Module, Clause).

compile_auxiliary_clause(Module, Clause) :-
    Clause = (Head:-Body),
    '$current_source_module'(SM),
    (   predicate_property(SM:Head, defined)
    ->  true
    ;   SM == Module
    ->  compile_aux_clauses([Clause])
    ;   compile_aux_clauses([Head:-Module:Body])
    ).

control((_,_)).
control((_;_)).
control((_->_)).
control((_*->_)).
control(\+(_)).
control($(_)).

is_aux_meta(Term) :-
    callable(Term),
    functor(Term, Name, _),
    sub_atom(Name, 0, _, _, '__aux_meta_call_').

compile_meta(CallIn, CallOut, M, Term, (CallOut :- Body)) :-
    replace_subterm(CallIn, true, Term, Term2),
    term_variables(Term2, AllVars),
    term_variables(CallIn, InVars),
    intersection_eq(InVars, AllVars, HeadVars),
    copy_term_nat(CallIn+HeadVars, NAT),
    variant_sha1(NAT, Hash),
    atom_concat('__aux_meta_call_', Hash, AuxName),
    expand_goal(CallIn, _Pos0, Body, _Pos, M, [], (CallOut:-CallIn), []),
    length(HeadVars, Arity),
    (   Arity > 256                 % avoid 1024 arity limit
    ->  HeadArgs = [v(HeadVars)]
    ;   HeadArgs = HeadVars
    ),
    CallOut =.. [AuxName|HeadArgs].

%!  replace_subterm(From, To, TermIn, TermOut)
%
%   Replace instances (==/2) of From inside TermIn by To.

replace_subterm(From, To, TermIn, TermOut) :-
    From == TermIn,
    !,
    TermOut = To.
replace_subterm(From, To, TermIn, TermOut) :-
    compound(TermIn),
    compound_name_arity(TermIn, Name, Arity),
    Arity > 0,
    !,
    compound_name_arity(TermOut, Name, Arity),
    replace_subterm_compound(1, Arity, From, To, TermIn, TermOut).
replace_subterm(_, _, Term, Term).

replace_subterm_compound(I, Arity, From, To, TermIn, TermOut) :-
    I =< Arity,
    !,
    arg(I, TermIn, A1),
    arg(I, TermOut, A2),
    replace_subterm(From, To, A1, A2),
    I2 is I+1,
    replace_subterm_compound(I2, Arity, From, To, TermIn, TermOut).
replace_subterm_compound(_I, _Arity, _From, _To, _TermIn, _TermOut).


%!  intersection_eq(+Small, +Big, -Shared) is det.
%
%   Shared are the variables in Small that   also appear in Big. The
%   variables in Shared are in the same order as Small.

intersection_eq([], _, []).
intersection_eq([H|T0], L, List) :-
    (   member_eq(H, L)
    ->  List = [H|T],
        intersection_eq(T0, L, T)
    ;   intersection_eq(T0, L, List)
    ).

member_eq(E, [H|T]) :-
    (   E == H
    ->  true
    ;   member_eq(E, T)
    ).

                 /*******************************
                 *      :- IF ... :- ENDIF      *
                 *******************************/

:- thread_local
    '$include_code'/3.

'$including' :-
    '$include_code'(X, _, _),
    !,
    X == true.
'$including'.

cond_compilation((:- if(G)), []) :-
    source_location(File, Line),
    (   '$including'
    ->  (   catch('$eval_if'(G), E, (print_message(error, E), fail))
        ->  asserta('$include_code'(true, File, Line))
        ;   asserta('$include_code'(false, File, Line))
        )
    ;   asserta('$include_code'(else_false, File, Line))
    ).
cond_compilation((:- elif(G)), []) :-
    source_location(File, Line),
    (   clause('$include_code'(Old, File, _), _, Ref)
    ->  erase(Ref),
        (   Old == true
        ->  asserta('$include_code'(else_false, File, Line))
        ;   Old == false,
            catch('$eval_if'(G), E, (print_message(error, E), fail))
        ->  asserta('$include_code'(true, File, Line))
        ;   asserta('$include_code'(Old, File, Line))
        )
    ;   throw(error(conditional_compilation_error(no_if, elif), _))
    ).
cond_compilation((:- else), []) :-
    source_location(File, Line),
    (   clause('$include_code'(X, File, _), _, Ref)
    ->  erase(Ref),
        (   X == true
        ->  X2 = false
        ;   X == false
        ->  X2 = true
        ;   X2 = X
        ),
        asserta('$include_code'(X2, File, Line))
    ;   throw(error(conditional_compilation_error(no_if, else), _))
    ).
cond_compilation(end_of_file, end_of_file) :-   % TBD: Check completeness
    !,
    source_location(File, _),
    (   clause('$include_code'(_, OF, OL), _)
    ->  (   File == OF
        ->  throw(error(conditional_compilation_error(
                            unterminated,OF:OL), _))
        ;   true
        )
    ;   true
    ).
cond_compilation((:- endif), []) :-
    !,
    source_location(File, _),
    (   (   clause('$include_code'(_, File, _), _, Ref)
        ->  erase(Ref)
        )
    ->  true
    ;   throw(error(conditional_compilation_error(no_if, endif), _))
    ).
cond_compilation(_, []) :-
    \+ '$including'.

'$eval_if'(G) :-
    expand_goal(G, G2),
    '$current_source_module'(Module),
    Module:G2.
