[R] help for nls

[BARNERIAS Cyrille]

Hello,

I am beginning with R and I would like to test a non linear model. But I do
not find exactly wath I am looking for in nls packages (or I do not know
where to search).
I would like to try a model like this : y=b * x exp(n)/(a exp(n) + x exp
(n))
Where 
a = a0 + a1z
b= b0 + b1z
x and z are variables
y the variable that I am trying to modelise
a0, a1, b0 and b1 are parameters to determine.
I am wondering if I can use nls and if so how do I have to write my command
?
Thanks in advance for your help.

Cyrille Barnérias
Adjoint au chef d'échelon interrégional de Caen
Inventaire forestier national
73 rue Marie Curie
14 200 Hérouville Saint-Clair
France
Tel : 02.31.47.71.53