docs: def mass_from_j (j_abs):
      """Estimate mass in cgs from absolute J magnitude, using the relationship of
      Delfosse+ (2000A&A...364..217D).
  
      j_abs - The absolute J magnitude.
  
      Returns: the estimated mass in grams.
  
      If j_abs > 11, a fixed result of 0.1 Msun is returned. Values of j_abs <
      5.5 are illegal and get NaN. There is a discontinuity in the relation at
      j_abs = 11, which yields 0.0824 Msun.
  
      """
      j_abs = np.asfarray (j_abs)
      return np.piecewise (j_abs,
                           [j_abs > 11,
                            j_abs <= 11,
                            j_abs < 5.5],
                           [0.1 * cgs.msun,
                            _delfosse_mass_from_j_helper,
                            np.nan])