--- a/src/Vector.hh	Mon Dec 08 00:36:24 2008 +0000
+++ b/src/Vector.hh	Mon Dec 08 01:08:00 2008 +0000
@@ -44,6 +44,9 @@
     _Vector operator-(const _Vector &v) const {
         return _Vector(this->x-v.x, this->y-v.y);
     }
+    _Vector operator- (void) const {
+        return _Vector(-this->x, -this->y);
+    }
     _Vector operator*(const T &scalar) const {
         return _Vector(this->x*scalar, this->y*scalar);
     }