code: def _process_extensions(exts):
      
      parts = [
          _process_form(form)
          for form in exts
      ]
      parts.sort()
      return b"".join(parts)+b"\x1c"
docs: 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`.