nonlinear fitting when both x and y having measurement error?

Dear r-help,

I want to conduct nonlinear fitting to a data frame having x and y
variables.  Because both x and y have measurement error, I want to
include error term of x variable in the model.  I'm not sure but I 
think ordinary nls model only consider error term of y variable.

How can I do this kind of nonlinear fitting in R.  Is there any
examples in nls package?

Thanks in advance,

Etsushi Kato
ekato at ees.hokudai.ac.jp
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