docs: def add_const(features):
      """
      Prepend the constant feature 1 as first feature and return the modified
      feature set.
  
      Parameters
      ----------
      features : ndarray or DataFrame
      """
      content = np.empty((features.shape[0], features.shape[1] + 1), dtype='float64')
      content[:, 0] = 1.
      if isinstance(features, np.ndarray):
          content[:, 1:] = features
          return content
      content[:, 1:] = features.iloc[:, :].values
      cols = ['Constant'] + features.columns.tolist()
      return pd.DataFrame(data=content, index=features.index, columns=cols, dtype='float64')