docs: def get_dual_variables(self):
          """
          Gets dual variables.
          
          Returns
          -------
          lam : vector
          nu : vector
          mu : vector
          pi : vector
          """
          
          if self.problem:
              return self.problem.recover_dual_variables(self.lam*self.obj_sca,
                                                         self.nu*self.obj_sca,
                                                         self.mu*self.obj_sca,
                                                         self.pi*self.obj_sca)
          else:
              return None,None,None,None