non-central t : R v.Splus

Hi

For bureaucratic reasons beyond my control I need to rewrite an R 
function
(for producing operating characteristic curves) as an Splus function 
(
version 6 , windows XP ).

The R function makes extensive use of the fact that the student's t
distribution function  pt()  has a non-centrality parameter built in 
...
sadly that parameter is not present in the Splus pt() function .

However, the Splus f distribution function pf() does have such a 
parameter
, so I have tried to write my own non-central version of pt() based 
around
pf() , using the relationship between the t and F distributions.

Unfortunately my success has been limited  ... I can only get 
correct
probabilities for part of the range of the quantile space , failing 
when
the quantile becomes small ... and I'm beginning to wonder whether 
it's
actually possible to do what I want at all , given that the range of 
x in
F(x) is [ 0:Inf ] while that in t(x) is [-Inf , Inf ] , and the
non-central t  distribution is not symmetric ...

Do any wiser heads than mine have any experience or advice to offer 
...
?

       thanks          Bob Kinley