[R] Passing parameters to 'optim' fn function

["José Luis Aznarte M."]

    Hi again! I'm using the 'optim' method to fix the parameters of a 
model. I have written the function to be minimised and another function 
which returns the gradient of the error. Now my problem is that, in 
order to compute that gradient, it would be extremely convenient to be 
able to pass some parameters to the gradient function. I don't see how 
to do it given the fixed syntax of 'optim', which does not allow for 
parameters being passed to fn and gr:

 > optim(par, fn, gr = NULL, ...)

    Of course the first idea would be to "pack" the extra parameters in 
the vector 'par', but in that case the extra parameters would be 
optimized too.
    Does anyone have an idea on how to pass parameters to gr in optim? 
Thanks for your time!