def S_id(v):
  """Fingerprints a potential value to a string identifier."""
  # This mostly "trims" the cosmological constant (so we can
  # always disambiguate by just adding more digits and keeping
  # substring identity), but nevertheless makes e.g.
  # 11.99999999999972 ==> S1200000.
  return 'S{:07d}'.format(int(-v * 1e5 + 1e-4))