docs: def c_overturned(step):
      """Theoretical overturned concentration.
  
      This compute the resulting composition profile if fractional
      crystallization of a SMO is assumed and then a purely radial
      overturn happens.
  
      Args:
          step (:class:`~stagpy.stagyydata._Step`): a step of a StagyyData
              instance.
      Returns:
          tuple of :class:`numpy.array`: the composition and the radial position
          at which it is evaluated.
      """
      rbot, rtop = misc.get_rbounds(step)
      cinit, rad = init_c_overturn(step)
      radf = (rtop**3 + rbot**3 - rad**3)**(1 / 3)
      return cinit, radf