should the fitted surface be computed exactly or via
interpolation from a kd tree? Can be abbreviated.
statistics
should the statistics be computed exactly or
approximately? Exact computation can be very slow. Can be abbreviated.
trace.hat
should the trace of the smoother matrix be computed
exactly or approximately? It is recommended to use the approximation
for more than about 1000 data points. Can be abbreviated.
cell
if interpolation is used this controls the accuracy of the
approximation via the maximum number of points in a cell in the kd
tree. Cells with more than floor(n*span*cell) points are subdivided.