{ \left ( I - \frac{v \otimes v}{|v|^0} \right ) }a \cdot { \left ( I - \frac{v \otimes v}{|v|^0} \right ) }\nabla _v{ \psi \left ( x, r \frac{v}{|v|}\right )}f \nonumber \\ & = - \int _{v \neq 0} { \left ( I - \frac{v \otimes v}{|v|^0} \right ) }a \cdot { \left ( I - \frac{v \otimes v}{|v|^0} \right ) }\nabla _v { \psi \left ( x, r \frac{v}{|v|}\right )}f \nonumber \\ & = - { \int _{{{{\bf}R}}^d \times r { {\bf}{S}}}\!\!\! { \left ( I - \frac0{r^0}({\omega \otimes \omega}) \right ) }a \cdot { \left ( I - \frac0{r^0}({\omega \otimes \omega}) \right ) }\nabla _v