--- "a/simple tree.hpp"
+++ "b/simple tree.hpp"
@@ -323,7 +323,7 @@
 	public:
 		compare_nodes(StrictWeakOrdering comp) : comp_(comp) {};
 
-		bool operator()(const tree_node *a, const tree_node *b) 
+		bool operator()(const tree_node *a, const tree_node *b) const
 		{
 			return comp_(a->data, b->data);
 		}
--- a/tree.hh
+++ b/tree.hh
@@ -422,7 +422,7 @@
 			public:
 				compare_nodes(StrictWeakOrdering comp) : comp_(comp) {};
 				
-				bool operator()(const tree_node *a, const tree_node *b) 
+				bool operator()(const tree_node *a, const tree_node *b) const
 					{
 					return comp_(a->data, b->data);
 					}
--- a/xinlin.hh
+++ b/xinlin.hh
@@ -332,7 +332,7 @@
 	public:
 		compare_nodes(StrictWeakOrdering comp) : comp_(comp) {};
 
-		bool operator()(const tree_node *a, const tree_node *b) 
+		bool operator()(const tree_node *a, const tree_node *b) const
 		{
 			return comp_(a->data, b->data);
 		}