That is similar to `xtol + 2*fabs(xmis)`, where `xmid` is the middle of the interval. The idea was that if the interval is smaller than that the bisecting it will lead a middle that will round to one of the ends, Giving an infinite loop. It could be made to depend on the dynamic interval, I suppose.

The `tol` selection is rather tricky, and I confess that I no longer remember all of the details.