diff --git a/include/klein/norm.h b/include/klein/norm.h
index bca38f3..72b1729 100644
--- a/include/klein/norm.h
+++ b/include/klein/norm.h
@@ -1,3 +1,16 @@
+#ifdef KLEIN_NORM_H
+#error file included twice
+#endif
+#define KLEIN_NORM_H
+
+
+#ifndef KLEIN_H
+#warning Please include klein/klein.h
+#endif
+
+void klein_normalize(struct klein * klein);
+
+#ifdef KLEIN_IMPLEMENT
 static inline
 void __calculate_normal( float *p1, float *p2, float *p3, float *normal, unsigned char n)
 { unsigned char i;
@@ -116,3 +129,4 @@ void klein_normalize(struct klein * klein)
 
 	free(norm_vec);
 }
+#endif