 $$\label{Equ00} { \int _{{{{\bf}R}}^d \times {{{\bf}R}}^d}\!\!\! \psi { \left \langle f \right \rangle } } = \int _{v \neq 0} \psi \left ( x, r { \frac{v}{|v|}}\right ) f,\;\;\psi \in { C^0_c ({{{\bf}R}}^d \times {{{\bf}R}}^d)}{}.$$  