docs: def postproc_mask (stats_result):
      """Simple helper to postprocess angular outputs from StatsCollectors in the
      way we want.
  
      """
      n, mean, scat = stats_result
  
      ok = np.isfinite (mean)
      n = n[ok]
      mean = mean[ok]
      scat = scat[ok]
  
      mean *= 180 / np.pi # rad => deg
      scat /= n # variance-of-samples => variance-of-mean
      scat **= 0.5 # variance => stddev
      scat *= 180 / np.pi # rad => deg
      return ok, mean, scat