ms can be used with the
multi-dimensional r2r interface, the interpretation of such a separable
product of transforms along each dimension is problematic.  For example,
consider a two-dimensional <code>n0</code> by <code>n1</code>, r2hc by r2hc
transform planned by <code>fftw_plan_r2r_2d(n0, n1, in, out, FFTW_R2HC,
FFTW_R2HC, FFTW_MEASURE)</code>.  Conceptually, FFTW first transforms the rows
(of size <code>n1</code>) to produce halfcomplex rows, and then transforms the
columns (of size <code>n0</code>).  H