The format-number() function produces garbage output
for numbers greater than about 1e19.
Applies to Saxon 7.x releases since the new
format-number() implementation was released. Note that
the old Java-based implementation is still available as
Test case added: numberformat60.
Source code fixed (to a degree). The output is now OK
in the first 18 or 19 places, it is still not as good
as the Java output beyond this point.