code: def _call(self, dx):
          
          x = self.point
  
          dx_norm = dx.norm()
          if dx_norm == 0:
              return 0
  
          scaled_dx = dx * (self.step / dx_norm)
  
          if self.method == 'backward':
              dAdx = self.operator(x) - self.operator(x - scaled_dx)
          elif self.method == 'forward':
              dAdx = self.operator(x + scaled_dx) - self.operator(x)
          elif self.method == 'central':
              dAdx = (self.operator(x + scaled_dx / 2) -
                      self.operator(x - scaled_dx / 2))
          else:
              raise RuntimeError('unknown method')
  
          return dAdx * (dx_norm / self.step)
docs: Return ``self(x)``.