class_docstr: A pointer used by propbank to identify one or more constituents in
  a parse tree.  ``PropbankPointer`` is an abstract base class with
  three concrete subclasses:
  
    - ``PropbankTreePointer`` is used to point to single constituents.
    - ``PropbankSplitTreePointer`` is used to point to 'split'
      constituents, which consist of a sequence of two or more
      ``PropbankTreePointer`` pointers.
    - ``PropbankChainTreePointer`` is used to point to entire trace
      chains in a tree.  It consists of a sequence of pieces, which
      can be ``PropbankTreePointer`` or ``PropbankSplitTreePointer`` pointers.
class_docstr_tok: ["A","pointer","used","by","propbank","to","identify","one","or","more","constituents","in","a","parse","tree",".","``","PropbankPointer","``","is","an","abstract","base","class","with","three","concrete","subclasses",":","-","``","PropbankTreePointer","``","is","used","to","point","to","single","constituents",".","-","``","PropbankSplitTreePointer","``","is","used","to","point","to","'split'","constituents",",","which","consist","of","a","sequence","of","two","or","more","``","PropbankTreePointer","``","pointers",".","-","``","PropbankChainTreePointer","``","is","used","to","point","to","entire","trace","chains","in","a","tree",".","It","consists","of","a","sequence","of","pieces",",","which","can","be","``","PropbankTreePointer","``","or","``","PropbankSplitTreePointer","``","pointers","."]
code: def __init__(self):
      if self.__class__ == PropbankPointer:
          raise NotImplementedError()
  
imports: import re
  from functools import total_ordering
  from xml.etree import ElementTree
  from nltk.corpus.reader.api import *
  from nltk.corpus.reader.util import *
  from nltk.internals import raise_unorderable_types
  from nltk.tree import Tree
label_desc: Use image node_id 1 to create a new PropbankPointer object  with example: obj = PropbankPointer()