def asymmetry (A,B,C):
  """
  Ray's asymmetry parameter for molecular rotation.
  For a prolate symmetric top (B = C), kappa = -1.
  For an oblate symmetric top (B = A), kappa = +1.
  See Townes and Schawlow, Ch. 4.
  """
  return (2.*B - A - C)/(A - C)