diff --git a/src/lib/between.pl b/src/lib/between.pl
index db87c45c..8aa8bca5 100644
--- a/src/lib/between.pl
+++ b/src/lib/between.pl
@@ -12,7 +12,7 @@ integers that may also be interesting.
 :- use_module(library(lists), [length/2]).
 :- use_module(library(error)).
 
-%% between(+Lower, +Upper, -X).
+%% between(+Lower, +Upper, ?X).
 %
 % Given Lower and Upper are both integer numbers, true iff X is an integer so that _Lower =< X =< Upper_.
 % Can be used both to check if X is between Lower and Upper or to generate an integer between