Here's what I don't like:

``
In [4]: from bipy.core.sequence import DNASequence, RNASequence, NucleotideSequence

In [9]: isinstance(DNASequence('AC'),NucleotideSequence)
Out[9]: True

In [12]: isinstance(NucleotideSequence('AC'),DNASequence)
Out[12]: False
``

Is ``NucleotideSequence('AC')`` really equal to ``DNASequence('AC')``? I don't think so.