[Python-ideas] Add OrderedSet now that OrderedDict is in collections

[Jan Kanis]

> So, let's review what we have in terms of data structures:
>
> Structure       Stable  Unique  Python type
> -------------------------------------------
> Multiset        no      no      -
> Set             no      yes     set
> Map             no      yes     dict
> List            yes     no      list, tuple
> Ordered set     yes     yes     -
> Ordered map     yes     yes     collections.OrderedDict
>
> where "stable" means that input order is retained.

<slightly off topic>

And where do Multimaps fit in? And Ordered Mulitmaps?
Example of an Ordered Multimap in the wild: the headers of an email
message, where multiple values may be attached to a single key, and
the ordering of key-value pairs is important (and not all key-value
pairs with the same key are necessarily adjacent). But basically it is
anything that behaves like a sequence of key-value pairs. Come to
think of it, 'headers' in lots of network protocols are like this.

(ps: +0 on adding ordered sets)