def get_signer_url(self, signer):
        """Return URL where signer signs document.

        Raise ``NotImplementedError`` in case the backend does not support
        "signer view" feature.

        Default implementation reverses :meth:`get_signer_url_name` with
        ``signer.pk`` as argument.

        """
        return reverse(self.get_signer_url_name(), args=[signer.pk])