bug#58929: 29.0.50; Calc: finding roots utpn doesn't work as advertised

[Mattias Engdegård]

Unfortunately straight Newton won't converge for this function (erf, 
essentially) if you stray too far from the origin (ie, the mean) so I suggest 
you always use that as a starting guess no matter what point you are trying to 
find the inverse for.

Another way is to use an interval as starting guess instead of a single point; 
that seems to make Calc switch tactics and persevere.

One thing that can happen is running out of stack, because many of the 
implementations like math-newton-root are recursive. This should be easy to 
remedy as they all seem to be tail calls.

Obviously the claim to guarantee a solution for "any initial guess" is fanciful.