[R] nls with some coefficients fixed

[nashjc at uottawa.ca]

For nls, the fixing (or masking) of parameters is not, to my knowledge,
possible.

This is something I've been trying to get in such routines for over 2
decades. Masks are
available, but not yet well documented, in Rcgmin and Rvmmin packages.
However, these use
an optim() style approach, which is quite different from nls(). If there's
sufficient
interest and some collaboration, I'd be willing to have a go at providing
such
functionality, but it would take quite a bit of work to provide the full
capability of nls().

JN