docs: def inv_logistic(y: Union[float, np.ndarray],
                   k: float,
                   theta: float) -> Optional[float]:
      r"""
      Inverse standard logistic function:
  
      .. math::
  
          x = ( log( \frac {1} {y} - 1) / -k ) + \theta
  
      Args:
          y: :math:`y`
          k: :math:`k`
          theta: :math:`\theta`
  
      Returns:
          :math:`x`
  
      """
      if y is None or k is None or theta is None:
          return None
      # noinspection PyUnresolvedReferences
      return (np.log((1 / y) - 1) / -k) + theta