Index: Doc/library/functions.rst
===================================================================
--- Doc/library/functions.rst	(Revision 81300)
+++ Doc/library/functions.rst	(Arbeitskopie)
@@ -1079,10 +1079,18 @@
 .. function:: sum(iterable[, start])
 
    Sums *start* and the items of an *iterable* from left to right and returns the
-   total.  *start* defaults to ``0``. The *iterable*'s items are normally numbers,
-   and are not allowed to be strings.  The fast, correct way to concatenate a
-   sequence of strings is by calling ``''.join(sequence)``.  To add floating
-   point values with extended precision, see :func:`math.fsum`\.
+   total. *start* defaults to ``0``. The *iterable*'s items are normally numbers.
+   *start* must not be a string.
+   
+   .. note::
+      Using sum to concatenate sequences will result in quadratic run-time performance.
+   
+   Alternatives:
+      - To concatenate an iterable of iterables, such as a list of lists, use
+        :func:`itertools.chain.from_iterable` and call the appropriate constructor,
+         such as :class: `list`, on the result.
+      - The fast, correct way to concatenate a sequence of strings is by calling 
+        ``''.join(sequence)`` (see :func:`str.join`).  
+      - To add floating point values with extended precision, see :func:`math.fsum`\.
 
 
 .. function:: super([type[, object-or-type]])