You'll need to have libfftw3 installed.  On my Linux machines, I needed
to configure that with --enable-shared.


The results we get are slightly different from those of Mark Newman's
stand-alone cart program. For example, when using uspop.dat data, we
get 1.002184 and 2.004368 for x = 1 and 2 (using a grid 0, 1, 2, ...)
whereas Mark's code yields 1.00214 and 2.00428.  This seems to be a
feature of using a different grid (1:1024 rather than 0:1024 and
similarly for y).  See tests/check.R.