'Serialize an individual item from the spine of the input document.
A reference to this item is stored in self.href_offsets'

def serialize_item(self, item):

    buf = self.buf
    if (not item.linear):
        self.breaks.append((buf.tell() - 1))
    self.id_offsets[urlnormalize(item.href)] = buf.tell()
    if item.is_section_start:
        buf.write('<a    ></a>    ')
    if item.is_article_start:
        buf.write('<a    ></a>    <a    ></a>')
    for elem in item.data.find(XHTML(u'body')):
        self.serialize_elem(elem, item)
    if self.write_page_breaks_after_item:
        buf.write('<mbp:pagebreak/>')
    if item.is_article_end:
        buf.write('<a    ></a>    <a    ></a>')
    if item.is_section_end:
        buf.write('    <a    ></a>')
    self.anchor_offset = None
