code: def _get_xk(self, yk):
          '''Compute approximate solution from initial guess and approximate
          solution of the preconditioned linear system.'''
          if yk is not None:
              return self.x0 + self.linear_system.Mr * yk
          return self.xf _get_xk(self, yk):
          '''Compute approximate solution from initial guess and approximate
          solution of the preconditioned linear system.'''
          if yk is not None:
              return self.x0 + self.linear_system.Mr * yk
          return self.x0
docs: Compute approximate solution from initial guess and approximate
          solution of the preconditioned linear system.