Add an example for math.fsum() and elaborate on the accurary note.
diff --git a/Doc/library/math.rst b/Doc/library/math.rst
index ab39424..55e03bf 100644
--- a/Doc/library/math.rst
+++ b/Doc/library/math.rst
@@ -80,14 +80,18 @@
.. function:: fsum(iterable)
Return an accurate floating point sum of values in the iterable. Avoids
- loss of precision by tracking multiple intermediate partial sums. The
- algorithm's accuracy depends on IEEE-754 arithmetic guarantees and the
- typical case where the rounding mode is half-even.
+ loss of precision by tracking multiple intermediate partial sums::
- .. note::
+ >>> sum([.1, .1, .1, .1, .1, .1, .1, .1, .1, .1])
+ 0.99999999999999989
+ >>> fsum([.1, .1, .1, .1, .1, .1, .1, .1, .1, .1])
+ 1.0
- The accuracy of fsum() may be impaired on builds that use
- extended precision addition and then double-round the results.
+ The algorithm's accuracy depends on IEEE-754 arithmetic guarantees and the
+ typical case where the rounding mode is half-even. On some non-Windows
+ builds, the underlying C library uses extended precision addition and may
+ occasionally double-round an intermediate sum causing it to be off in its
+ least significant bit.
.. function:: isinf(x)