a generator yielding the :class:`Message` structures in the index

def messages(self):
        '''a generator yielding the :class:`Message` structures in the index'''

        # the file contains the fixed-size file header followed by
        # fixed-size message structures. start after the file header and
        # then simply return the message structures in sequence until the
        # end of the file.

        offset = self.LENGTH
        while offset < len(self.mmap):
            yield Message(mm=self.mmap, offset=offset)
            offset += Message.LENGTH