<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def rsdl_sn(self, U):
 """Compute dual residual normalisation term. Overriding this method is required if methods :meth:`cnst_A`, :meth:`cnst_AT`, :meth:`cnst_B`, and :meth:`cnst_c` are not overridden. """


        return self.rho * np.linalg.norm(self.cnst_AT(U))