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:- module(tau, ['$member'/2, '$findall'/4, '$bagof'/3, '$setof'/3, '$if'/3]).
:- meta_predicate
'$findall'(?, 0, -, ?),
'$bagof'(?, ^, -),
'$setof'(?, ^, -),
'$if'(0, 0, 0).
%! '$member'(?Element, ?List)
%
% True if Element is a member of List.
'$member'(X, [X|_]).
'$member'(X, [_|Xs]) :- '$member'(X, Xs).
%! '$findall'(+Template, :Goal, -Bag, +Tail)
%
% True if Bag is a list of values in the form Template that would make the
% goal Goal succeed. Tail is the tail of the difference list Bag-Tail.
'$findall'(Template0, Goal0, Instances, Tail) :-
copy_term(Template0-Goal0, Template1-Goal1),
call(Goal1),
'$push_global_stack'(Var, Template1),
false ; '$flush_global_stack'(Var, Instances, Tail).
%! '$bagof'(+Template, :Goal, -Bag)
%
% Unify Bag with the alternatives of Template for the goal Goal.
'$bagof'(Template, Goal0, Answer) :-
'$free_variable_set'(Template^Goal0, Goal1, FV),
findall(FV-Template, Goal1, Answers, []),
keygroup(Answers, KeyGroups),
keysort(KeyGroups, KeySorted),
'$member'(FV-Answer, KeySorted).
%! '$setof'(+Template, :Goal, -Bag)
%
% Equivalent to '$bagof'/3, but sorts the result using sort/2 to get a sorted
% list of alternatives without duplicates.
'$setof'(Template, Goal0, Answer) :-
'$free_variable_set'(Template^Goal0, Goal1, FV),
findall(FV-Template, Goal1, Answers, []),
keygroup(Answers, KeyGroups),
keysort(KeyGroups, KeySorted),
'$member'(FV-Unsorted, KeySorted),
sort(Unsorted, Answer).
%! '$if'(:Condition, :Action, :Else)
%
% This construct implements the so-called soft-cut.
'$if'(If, Then, Else) :-
( call(If),
'$push_global_stack'(Stack, _),
call(Then)
; '$flush_global_stack'(Stack, [], []),
call(Else)
).
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