and backrefs can't be used inside []

Closes #3011.

* Expand comments.
* Swap variable names in the sum_exact code so that x and y
  are consistently chosen as the larger and smaller magnitude
  values respectively.

Issue 2855: Fix obscure crasher by slowing down the entire module.  Mimics what was done to dictionaries in r59223.

all the upper level libraries that use it, including urllib2.
Added and fixed some tests, and changed docs correspondingly.
Thanks to John J Lee for the patch and the pusing, :)

Works on issue #2877. Thanks Quentin Gallet-Gilles for the patch.