bah, I'm torn. I like keeping the ``SequenceIterator`` independent of the ``BiologicalSequence`` as including it makes this code more complex as the ``BiologicalSequence`` has no notion of quality scores. The issue is that, casting to ``BiologicalSequence`` then decouples sequence from qual, which makes it more awkward to express transforms like reverse complement. Leaving this cast up to the developer, or the ``success_f`` callback for ``Workflow`` is an easy option which keeps this code more lean and mean.

@jrrideout @gregcaporaso, do you think it would be useful to include quality in the ``BiologicalSequence`` object?