[R] little help with Nonlinear regression needed

[Paka yag]

  Hello

as the subject says I need a little help with following nonlinear regression

y(t) = a + b*x(t-1)+c*y_(t-1)+G*(a+b*x_(t-1)) 

where G=[1 + exp(-s(x_(t-1) - k)]^(-1)

(In reality there are more variables)

Please could anybody give me a hint how I can estimate this?? 
Should I use nls()? what parameter settings would I use then?

And one more: how would I estimate it if the independant variables werent lagged, would it make a difference?

Your help is much appreciated!!



Thank you

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