99 Bottles of Beer/Prolog: Difference between revisions

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 =={{header|Prolog}}==
 {{works with|SWI Prolog}}
-<lang prolog>bottles(0):-!.
+<lang prolog>bottles(0).
 bottles(X):-
     writef('%t bottles of beer on the wall \n',[X]),
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 :- bottles(99).</lang>
+It is possible to extend the previous version so to handle singular and plurals of the word 'bottle':
-An other version that handles plural/not plural conditions.
+<lang prolog>bottles(0):- write('no more bottles of beer').
+bottles(B) :-
+    dif(B,0), NewB is B - 1,
+    (
+        dif(B,1), writef('%t bottles of beer on the wall \n',[B]), writef('%t bottles of beer\n',[B]);
+        writef('%t bottle of beer on the wall \n',[B]), writef('%t bottle of beer\n',[B])
+    ),
+    write('Take one down, pass it around\n'),
+    (
+        NewB > 1, writef('%t bottles of beer on the wall \n\n',[NewB]);
+        NewB =:= 1, writef('%t bottle of beer on the wall \n\n',[NewB]);
+        true
+    ),
+    bottles(NewB).
+:- bottles(99).</lang>
+Another version that handles plural/not plural conditions.
 <lang prolog>line1(X):- line2(X),write(' on the wall').