Question: UML 1 uses the name Collaboration Diagram. UML 2 uses the name Communication Diagram. [In this link you can find the definition and one nice example](http://www.agilemodeling.com/artifacts/communicationDiagram.htm). [Here you can find a nice reference about how to create them](http://www.uml-diagrams.org/communication-diagrams-reference.html).

Unfortunately, I didn't find this diagram in the software I have been using lately: [DIA](http://dia-installer.de/). I think it is not available in [WhiteStarUML](http://sourceforge.net/projects/whitestaruml/) either. There is a tutorial in the help, but I wasn't able to make it work. As WhiteStarUML is a fork of StartUML, maybe it is just not done yet.

![WhiteStarUML Screenshot](https://i.stack.imgur.com/RzWlC.jpg)

*WhiteStarUML Screenshot: there is a folder for Collaboration Diagram but not a file inside to double click it to create it (like the Class Diagrams or UseCase Diagrams)*

What your favourite free tool to create those diagrams?

Answer: 
Tried reinstall, but bug not gone.

Then, I open `pytz/__init__.py`, add a line `zone = 'UTC'`, problem is gone:

```
zone = _unmunge_zone(zone)
zone = 'UTC'
if zone not in _tzinfo_cache:
    if zone in all_timezones_set:
        fp = open_resource(zone)
        try:
            _tzinfo_cache[zone] = build_tzinfo(zone, fp)
        finally:
            fp.close()
    else:
        raise UnknownTimeZoneError(zone)

```

This is quick and simple solution, but you'd better find the real problem( version? system? ) if you have time.
Look into pytz/**init**.py and see if your zone exists.
In my case 'America/Argentina/Buenos\_Aires'.