15.37.18  NONLINEAR   

Selects nonlinear fitting techniques.  If ON MINUIT is used to do the fit.
The ECHISQ, GRADIENT, TOLERENCE, REPEAT, and PRINTLEVEL are used to modify
the MINUIT fit.  They are ignored for linear fits.   If  you  specify  the
option MINUIT you will enter interactive MINUIT mode.  You must then guide
the fit using MINUIT commands.  In addition you may  specify  the  maximum
and  minimum  value  for  each  coeffiecint with the CMINIMUM and CMAXIMUM
options.   You  might  also  wish  to  pick  an  initial  error  for  each
coefficinent, and possibly an initial value.

The   nonlinear  errors  agree  with  the  linear  values  if  you  select
ECHISQ=1.0.   Note:   Though  Topdrawer  and  PAW  both  use  MINUIT,  the
resulting  CHISQ  disagrees  by  about a factor of 3.  The TOPDRAWER CHISQ
seems to agree with the correct value by subtracting the fitted curve from 
the data and calculating SUM((Y/DY)**2)/(Points-Params)

The  linear  methods  are  much faster and convergence is guaranteed if no
numerical errors occurr.  Unfortunately many curves can not  be  expressed
as a linear combination of terms.

Most  linear  fits may be performed either by linear or nonlinear methods.
This may be useful in checking the accuracy of the fitting techniques.
(Default:OFF, ON if GAUSSIAN or EXPONENTAIL)