diff --git a/Geo/SVector3.h b/Geo/SVector3.h
index 815752dc121af11357b1da7efec4792697726f52..532389c796801c6c423760a3f0ad356823b55737 100644
--- a/Geo/SVector3.h
+++ b/Geo/SVector3.h
@@ -128,6 +128,9 @@ inline SVector3 operator+(const SVector3 &a,const SVector3 &b)
 inline SVector3 operator-(const SVector3 &a,const SVector3 &b)
 { return SVector3(a[0] - b[0], a[1] - b[1], a[2] - b[2]); }
 
+inline SVector3 operator-(const SVector3 &a)
+{ return SVector3(-a[0], -a[1], -a[2]); }
+
 
 inline void buildOrthoBasis_naive(SVector3 &dir, SVector3 &dir1, SVector3 &dir2)
 {