diff --git a/A/queens.P b/A/queens.P
index c2d6170..25be237 100644
--- a/A/queens.P
+++ b/A/queens.P
@@ -1,3 +1,10 @@
+between(I, J, K) :-
+    I =< J,
+    (   I = K
+    ;   I1 is I+1,
+        between(I1, J, K)
+    ).
+
 legal_queens(N, Row, []) :- N =:= Row - 1.
 legal_queens(N, Row1, [(Row1, Col1)|Queens]) :-
     Row1 < N + 1,