the early work, the
nominal model was taken
to be a histogram from Monte Carlo simulations, but in this
implementation it is generalized to an
arbitrary PDF (which includes a RooHistPdf).  The algorithm basically
consists of a 
hypothesis test of an nth-order correction (null) against a n+1-th order
correction (alternate). 
The quantity q = -2 log LR is used to determine whether the n+1-th order
correction is a major 
improvement to the n-th order correction.  The distribution of q is
expected to be ro