docs: def logistic(x: Union[float, np.ndarray],
               k: float,
               theta: float) -> Optional[float]:
      r"""
      Standard logistic function.
  
      .. math::
  
          y = \frac {1} {1 + e^{-k (x - \theta)}}
  
      Args:
          x: :math:`x`
          k: :math:`k`
          theta: :math:`\theta`
  
      Returns:
          :math:`y`
  
      """
      # https://www.sharelatex.com/learn/List_of_Greek_letters_and_math_symbols
      if x is None or k is None or theta is None:
          return None
      # noinspection PyUnresolvedReferences
      return 1 / (1 + np.exp(-k * (x - theta)))