'Write links to external workbooks'

def _write_external_links(self, archive):

    wb = self.workbook
    for (idx, book) in enumerate(wb._external_links, 1):
        el = write_external_link(book.links)
        rel = write_external_book_rel(book)
        archive.writestr('{0}/externalLinks/externalLink{1}.xml'.format(PACKAGE_XL, idx), tostring(el))
        archive.writestr('{0}/externalLinks/_rels/externalLink{1}.xml.rels'.format(PACKAGE_XL, idx), tostring(rel))
