diff --git a/pmc/include/vector.h b/pmc/include/vector.h
--- a/pmc/include/vector.h
+++ b/pmc/include/vector.h
@@ -18,12 +18,12 @@
     int X ();
     int Y ();
 
-    Vector operator+ (Vector);
-    Vector operator- (Vector);
+    Vector operator+ (Vector&);
+    Vector operator- (Vector&);
     double abs ();
 
     void vector ();
-    double angle (Vector);
+    double angle (Vector&);
     void setMode (Mode m = RAD);
 
   private: