Provides tools for working with nonlinear least squares problems.
For the estimation of models reliable and robust tools than nls(), where the
the Gauss-Newton method frequently stops with 'singular gradient' messages.
This is accomplished by using, where possible, analytic derivatives to compute
the matrix of derivatives and a stabilization of the solution of the estimation
equations. Tools for approximate or externally supplied derivative matrices
are included. Bounds and masks on parameters are handled properly.