non-hierarchical data used to identify the resource. Keys are
        always strings. Values are strings when present, or None when
        missing.

        For more operations on the mapping, see
        :meth:`~hyperlink.URL.get()`, :meth:`~hyperlink.URL.add()`,
        :meth:`~hyperlink.URL.set()`, and
        :meth:`~hyperlink.URL.delete()`.
        )