diff --git a/src/hull.h b/src/hull.h
--- a/src/hull.h
+++ b/src/hull.h
@@ -1,3 +1,6 @@
+#ifndef _HULL_H_
+#define _HULL_H_
+
 // Class point needed by 2D convex hull code
 class Point {
 
@@ -11,7 +14,11 @@ public:
   }
   float x,y;
 
+
 };
 
+// required to sort points (e.g. Qt's qSort())
+bool operator<(const Point & p1, const Point & p2);
+
 int chainHull_2D( Point* P, int n, Point* H );
-
+#endif