power function using gnm()

Hi,

I'm using the package gnm to perform non-linear models and I cannot  
find how to write the formula for a power function model.
The equation is something like (1)   y ~ A * x^B   or  (2)    y ~ A *  
B^x.
I guess it is using the nonlin function Mult(), but couldn't find how.

Thank you for your help,

Best regards,
Natalia