Hey,

I've noticed that Sequence.positions throws a TypeError when iterating through DescriptiveMarker objects. Here's the line:

https://github.com/markreidvfx/pyaaf/blob/master/aaf/component/Sequence.pyx#L106

If 'component' is a DescriptiveMarker, the 'length' attribute will be None, so it errors on the second iteration with "0 += None".

~Travis