remove vestigial prolog/ directory (#444)

src/examples/domain.pl

@@ -0,0 +1,51 @@
+/* From the SICSTUS Prolog documentation at:
+https://sicstus.sics.se/sicstus/docs/3.7.1/html/sicstus_17.html
+*/
+
+:- module(domain, [domain/2]).
+
+:- use_module(library(atts)).
+:- use_module(library(ordsets), [
+        ord_intersection/3,
+        ord_intersect/2,
+        list_to_ord_set/2
+   ]).
+
+:- attribute dom/1.
+
+verify_attributes(Var, Other, Goals) :-
+        get_atts(Var, dom(Da)), !,          % are we involved?
+        (   var(Other) ->                   % must be attributed then
+            (   get_atts(Other, dom(Db)) -> %   has a domain?
+                ord_intersection(Da, Db, Dc),
+                Dc = [El|Els],              % at least one element
+                (   Els = [] ->             % exactly one element
+                    Goals = [Other=El]      % implied binding
+                ;   Goals = [],
+                    put_atts(Other, dom(Dc))% rescue intersection
+                )
+            ;   Goals = [],
+                put_atts(Other, dom(Da))    % rescue the domain
+            )
+        ;   Goals = [],
+            ord_intersect([Other], Da)      % value in domain?
+        ).
+verify_attributes(_, _, []).                % unification triggered
+                                            % because of attributes
+                                            % in other modules
+
+attribute_goals(Var, domain(Var,Dom)) :-     % interpretation as goal
+        get_atts(Var, dom(Dom)).
+
+domain(X, Dom) :-
+        var(Dom), !,
+        get_atts(X, dom(Dom)).
+domain(X, List) :-
+        list_to_ord_set(List, Set),
+        Set = [El|Els],                     % at least one element
+        (   Els = [] ->                     % exactly one element
+            X = El                          % implied binding
+        ;   put_atts(Fresh, dom(Set)),
+            X = Fresh                       % may call
+                                            % verify_attributes/3
+        ).