[R] obsolete(?) nls package

[John C Nash]

I am building some tools to carry out nonlinear least squares when the residuals are very
small (something that nls specifically is NOT designed to do). However, I wanted to put a
cross reference in the Rd file. Running R CMD check pkg gave a strange warning

  Obsolete package(s) 'nls' in Rd xrefs.

Has nls been deprecated? I suspect not, but clearly the warning is of interest.

JN