Nonlinear Functions
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 -- NLFunc: (void)
 -- NLFunc: (const NONLINEAR_FCN)
 -- NLFunc: (const NONLINEAR_FCN, const JACOBIAN_FCN)
 -- NLFunc: (const NLFunc &A)

 -- NLFunc&: operator = (const NLFunc &A)

 -- nonlinear_fcn: function (void) const;

 -- NLFunc&: set_function (const nonlinear_fcn F)

 -- jacobian_fcn: jacobian_function (void) const;

 -- NLFunc&: set_jacobian_function (const jacobian_fcn J)