What is the most efficient way to compare similar contents in two lists?

[geremy condra]

On Mon, Jun 13, 2011 at 9:30 AM, Chris Angelico <rosuav at gmail.com> wrote:
> On Tue, Jun 14, 2011 at 2:04 AM, Steven D'Aprano
> <steve+comp.lang.python at pearwood.info> wrote:
>> On Tue, 14 Jun 2011 01:39:50 +1000, Chris Angelico wrote:
>>
>>> Python: "Batteries Included".
>>>
>>> (Although Python 3 is "Most of the batteries you're used to, included".)
>>
>> Oh gods, you're not going to bring up sort(cmp=...) again are you????
>>
>> /me ducks and covers
>
> Ha! That *lengthy* thread started fairly soon after I joined this
> list. It was highly... informative. I learned a bit about Python, and
> a lot about python-list, from the posts there.

So what are we talking about here? Reduce?

Geremy Condra