Files solution.h, solution.cpp contain a class for solving for x in y = f(x) where x is a one-dimensional continuous monotonic function. This is not a matrix thing at all but is included because it is a useful thing and because it is a simpler version of the technique used in the non-linear least squares.
Files newmatnl.h, newmatnl.cpp contain a series of classes for non-linear least squares, to be extended to maximum likelihood in a later release.
Documentation for both of these is in the definition files. Simple examples are in sl_ex.cpp and nl_ex.cpp.
These packages do not work with the AT&T and HPUX compilers and newmatnl is questionable under Sun 4.0.1.