docs: def populate_unpaired_line(d_vals, f_f_header, missing_val=None):
    """
    used when a value in d_vals doesn't match anything in the other file.
  
    :return: a dictionary, indexed by key value, with the correct missing values
             populated for the other file.
    """
    if missing_val is None:
      raise MissingValueError("Need missing value to output " +
                              " unpaired lines")
    if f_f_header is not None:
      f_f_flds = [dict(zip(f_f_header, [missing_val] * len(f_f_header)))]
    else:
      assert(len(d_vals) > 0)
      f_f_num_cols = len(d_vals[d_vals.keys()[0]][0])
      f_f_flds = [[missing_val] * f_f_num_cols]
    return f_f_flds