docs: def count_series(y_true, y_score, countna=False):
      """
      Returns series whose i-th entry is the number of examples in the top i
      """
      y_true, y_score = to_float(y_true, y_score)
      top = _argsort(y_score)
  
      if not countna:
          a = (~np.isnan(y_true[top])).cumsum()
      else:
          a = range(1, len(y_true)+1)
  
      return pd.Series(a, index=range(1, len(a)+1))