old_contents: from sklearn.tree import DecisionTreeRegressor
  from sklearn.linear_model import LinearRegression, Ridge
  
  
  def default_tree_learner(depth=3):
      return DecisionTreeRegressor(
          criterion='friedman_mse',
          min_samples_split=2,
          min_samples_leaf=1,
          min_weight_fraction_leaf=0.0,
          max_depth=depth,
          splitter='best')
  
  def default_linear_learner(alpha=1):
      return Ridge(alpha = 1)
  
new_contents: from sklearn.tree import DecisionTreeRegressor
  from sklearn.linear_model import Ridge
  
  
  def default_tree_learner(depth=3):
      return DecisionTreeRegressor(
          criterion='friedman_mse',
          min_samples_split=2,
          min_samples_leaf=1,
          min_weight_fraction_leaf=0.0,
          max_depth=depth,
          splitter='best')
  
  
  def default_linear_learner(alpha=1):
      return Ridge(alpha = 1)
  
subject: delete unused import LinearRegression