def _sortkey(self, key='<STR_LIT:id>', language='<STR_LIT>'):
if key == '<STR_LIT:id>':<EOL><INDENT>return str(self.id)<EOL><DEDENT>elif key == '<STR_LIT>':<EOL><INDENT>return self.uri if self.uri else '<STR_LIT>'<EOL><DEDENT>else:<EOL><INDENT>l = label(self.labels, language, key == '<STR_LIT>')<EOL>return l.label.lower() if l else '<STR_LIT>'<EOL><DEDENT>
Provide a single sortkey for this collection.

:param string key: Either `id`, `uri`, `label` or `sortlabel`.
:param string language: The preferred language to receive the label in
    if key is `label` or `sortlabel`. This should be a valid IANA language tag.
:rtype: :class:`str`