code: def rsdl_sn(self, U):
          
  
          return self.rho * np.linalg.norm(self.cnst_AT(U))
docs: 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.