Non-linear regression with latent variable

Hi 
Can anyone please suggest me a package where I can estimate a non-linear
regression model? One of the independent variables is latent or unobserved.
I have an indicator variable for this unobserved variable; however the
relationship is known to be non-linear also. In terms of equations my
problem is

y=f(latent, fixed) 
q=g(latent) where q is the indicator variable

For me both f and g are non-linear.

Thanks
Samiul Hasan

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