Parses the sequence and atomic mass.

def _parse_sequence_tag(self):
        '''Parses the sequence and atomic mass.'''
        #main_tags = self._dom.getElementsByTagName("uniprot")
        #assert(len(main_tags) == 1)
        #entry_tags = main_tags[0].getElementsByTagName("entry")
        #assert(len(entry_tags) == 1)
        #entry_tags[0]

        entry_tag = self.entry_tag
        # only get sequence tags that are direct children of the entry tag (sequence tags can also be children of entry.comment.conflict)
        sequence_tags = [child for child in entry_tag.childNodes if child.nodeType == child.ELEMENT_NODE and child.tagName == 'sequence']
        assert(len(sequence_tags) == 1)
        sequence_tag = sequence_tags[0]

        # atomic mass, sequence, CRC64 digest
        self.atomic_mass = float(sequence_tag.getAttribute("mass"))
        self.sequence = "".join(sequence_tag.firstChild.nodeValue.strip().split("\n"))
        self.sequence_length = int(sequence_tag.getAttribute("length"))
        self.CRC64Digest = sequence_tag.getAttribute("checksum")