Blurb::
Absolute convergence tolerance
Description::
\c absolute_conv_tol (NL2SOL's \c
afctol) is the absolute function convergence tolerance (stop when half
the sum of squares is less than \c absolute_conv_tol, which is mainly
of interest on zero-residual test problems).

The internal default is a function of machine epsilon 
(as limited by \c function_precision). The default is selected with a value of -1.

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