code: def _check_package(pkg_xml, zipfilename, zf):
    
    # The filename must patch the id given in the XML file.
    uid = os.path.splitext(os.path.split(zipfilename)[1])[0]
    if pkg_xml.get('id') != uid:
      raise ValueError('package identifier mismatch (%s vs %s)' %
               (pkg_xml.get('id'), uid))
  
    # Zip file must expand to a subdir whose name matches uid.
    if sum( (name!=uid and not name.startswith(uid+'/'))
        for name in zf.namelist() ):
      raise ValueError('Zipfile %s.zip does not expand to a single '
               'subdirectory %s/' % (uid, uid))
docs: Helper for ``build_index()``: Perform some checks to make sure that
    the given package is consistent.