Blurb::
False convergence tolerance
Description::
\c false_conv_tol (NL2SOL's \c xftol) is the
false-convergence tolerance (stop with a suspicion of discontinuity
when a more favorable stopping test is not satisfied and a step of
scaled length at most \c false_conv_tol is not accepted)  

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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