docs: def spheres_sep(ar, aR, br, bR):
      """Return the separation distance between two spheres.
  
      Parameters
      ----------
      ar, br: array-like, shape (n,) in n dimensions
          Coordinates of the centres of the spheres `a` and `b`.
      aR, bR: float
          Radiuses of the spheres `a` and `b`.
  
      Returns
      -------
      d: float
          Separation distance.
          A negative value means the spheres intersect each other.
      """
      return vector.vector_mag(ar - br) - (aR + bR)