The Wigner Kernel of a Particle obtained from the Wigner Kernel of a   Spin by Group Theoretical Contraction
  The Moyal formalism for a particle can be derived from the Moyal formalism for a spin. This is done by contracting the group of rotations to the oscillator group. A new derivation is given for the contraction of the spin Wigner-kernel to the Wigner kernel of a particle. 
Jean-Pierre Amiet and Stefan Weigert