code: def _frozen_class(self):
      return ImmutableProbabilisticTree
  
imports: from nltk.probability import ProbabilisticMixIn
  from nltk.tree.parented import MultiParentedTree, ParentedTree
  from nltk.tree.tree import Tree
label_desc: Use image node_id 2 for calling the ImmutableProbabilisticTree obj's underlying member method code with example usage: obj._frozen_class() and returns: ImmutableProbabilisticTree