15.37.13  USER        

     USER=n
Does  a  fit to a user supplied function type.  N must be between 1 and 99
with a default of 1.  If  none  then  a  polynomial  is  used.   For  each
coefficient of the linear fit the user supplied function is called:
     Y=TDUFUN(n,NORD,X)
Where  n  is  the  function  type.   NORD is the number of the coefficient
(1-19) and X is the x value.  The user function must return the y value of
the term.

For  example  if  you  want  USER=1  to select a polynomial, and USER=2 to
select an inverse polynomial.  You would write the following subroutine:
     FUNCTION TDUFUN(n,NORD,X)
     IF (n.eq.1) THEN
       TDUFUN=X**NORD
     IF (n.eq.2) THEN
       TDUFUN=X**(-NORD)
     ELSE
     END