code: def is_fit(self) -> bool:
      return isinstance(self.booster, Booster)
  
imports: from collections import defaultdict
  from typing import List, Mapping
  import numpy
  import pandas
  from lightgbm.basic import Booster
  from dtreeviz.models.shadow_decision_tree import ShadowDecTree, VisualisationNotYetSupportedError
  from dtreeviz import utils
label_desc: Use image node_id 3 for calling the ShadowLightGBMTree obj's underlying member method code with example usage: obj.is_fit() and returns: isinstance