diff --git a/src/qalculate_tde_utils.h b/src/qalculate_tde_utils.h
index e51d0c1..e0c9238 100644
--- a/src/qalculate_tde_utils.h
+++ b/src/qalculate_tde_utils.h
@@ -41,7 +41,7 @@ struct tree_struct {
 	std::list<tree_struct> items;
 	std::list<tree_struct>::iterator it;
 	std::list<tree_struct>::reverse_iterator rit;
-	std::vector<void*> objects;	
+	std::vector<void*> objects;
 	tree_struct *parent;
 	void sort() {
 		items.sort();
@@ -49,9 +49,10 @@ struct tree_struct {
 			it->sort();
 		}
 	}
-	bool operator < (tree_struct &s1) const {
-		return item < s1.item;	
-	}	
+	bool operator < (const tree_struct &s1) const
+	{
+		return item < s1.item;
+	}
 };
 struct mode_struct {
 	PrintOptions po;