Blurb::
Singular convergence tolerance
Description::
``singular_conv_tol``
(NL2SOL's ``sctol``) is the singular convergence tolerance, which works
in conjunction with :dakkw:`method-nl2sol-singular_radius` to test for
underdetermined least-squares problems (stop when the relative
reduction yet possible in the sum of squares appears less then
``singular_conv_tol`` for steps of scaled length at most :dakkw:`method-nl2sol-singular_radius`).

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