code: def rsdl_s(self, Yprev, Y):
          
  
          return self.rho * self.cnst_AT(Yprev - Y)
docs: Compute dual residual vector.
  
          Overriding this method is required if methods :meth:`cnst_A`,
          :meth:`cnst_AT`, :meth:`cnst_B`, and :meth:`cnst_c` are not
          overridden.