def _process_extensions(exts):
    """
    Generate the `Extensions String` from an iterable of data forms.

    :param exts: The data forms to generate the extensions string from.
    :type exts: :class:`~collections.abc.Iterable` of
        :class:`~.forms.xso.Data`
    :return: The `Extensions String`
    :rtype: :class:`bytes`

    Generate the `Extensions String` from the given `exts` as specified
    in :xep:`390`.
    """
    parts = [
        _process_form(form)
        for form in exts
    ]
    parts.sort()
    return b"".join(parts)+b"\x1c"