A solver's `Solve()`
method returns an estimate
of the model for a
given operator of compatible type^{},
a data input vector , and
possibly a starting model vector .
Usually, a programmer sets the solver's
convergence criteria. For example, geophysicists
usually combine a maximal number of iterations (for iterative solvers)
or an acceptable residual norm. Currently, HCL includes about three solver
classes. We hope that as more solvers are implemented,
a solver class hierarchy will develop.

11/11/1997