def get_signer_return_url(self, signer):
        """Return absolute URL where signer is redirected after signing.

        The URL must be **absolute** because it is typically used by external
        signature service: the signer uses external web UI to sign the
        document(s) and then the signature service redirects the signer to
        (this) `Django` website.

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

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

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