 \\ & = { \int _{{{{\bf}R}}^d \times {{{\bf}R}}^d}\!\!\! \psi (x,v) { \mathrm{div}_v}\left \{ { \mathrm{div}_v}\left [ f { \left ( I - \frac{v \otimes v}{|v|^0} \right ) }\right ] + 0 f \frac{v}{|v|^0}  \right \}