docs: def cdist_sq_periodic(ra, rb, L):
      """Return the squared distance between each point in on set,
      and every point in a second set, in periodic space.
  
      Parameters
      ----------
      ra, rb: float array-like, shape (n, d) and (m, d) in d dimensions.
          Two sets of points.
      L: float array, shape (d,)
          System lengths.
  
      Returns
      -------
      cdist_sq: float array-like, shape (n, m, d)
          cdist_sq[i, j] is the squared distance between point j and point i.
      """
      return np.sum(np.square(csep_periodic(ra, rb, L)), axis=-1)