code: def head_circumference(return_X_y=True):
      
      # y is real
      # recommend ExpectileGAM
      head = pd.read_csv(PATH + '/head_circumference.csv', index_col=0).astype(float)
      if return_X_y:
          y = head['head'].values
          X = head[['age']].values
          return _clean_X_y(X, y)
      return head
docs: head circumference for dutch boys
  
      Parameters
      ----------
      return_X_y : bool,
          if True, returns a model-ready tuple of data (X, y)
          otherwise, returns a Pandas DataFrame
  
      Returns
      -------
      model-ready tuple of data (X, y)
          OR
      Pandas DataFrame
  
      Notes
      -----
      X contains the age in years of each patient.
  
      y contains the head circumference in centimeters