  For the continuous Wigner function and for certain discrete Wigner functions,
permuting the values of the Wigner function in accordance with a symplectic
linear transformation is equivalent to performing a certain unitary
transformation on the state. That is, performing this unitary transformation is
simply a matter of moving Wigner-function values around in phase space. This
result holds in particular for the simplest discrete Wigner function defined on
a $d \times d$ phase space when the Hilbert-space dimension $d$ is odd. It does
not hold for a $d \times d$ phase space if the dimension is even. Here we show,
though, that a generalized version of this correspondence does apply in the
case of a two-qubit phase space. In this case, a symplectic linear permutation
of the points of the phase space, together with a certain reinterpretation of
the Wigner function, is equivalent to a unitary transformation.
