t); 'prod'. In multivariate
         problems, 'prod' uses a simplified product model which
         speeds up computations. 
 
  'acri' Criterion for adaptive bandwidth selection.
 

  Derivative Estimation.
  Generally I recommend caution when using derivative estimation
  (and especially higher order derivative estimation) -- can you
  really estimate derivatives from noisy data? Any derivative
  estimate is inherently more dependent on an assumed smoothness
  (expressed through the bandwidth