class_docstr: A string-based encoding of a particular chunking of a text.
  Internally, the ``ChunkString`` class uses a single string to
  encode the chunking of the input text.  This string contains a
  sequence of angle-bracket delimited tags, with chunking indicated
  by braces.  An example of this encoding is::
  
      {<DT><JJ><NN>}<VBN><IN>{<DT><NN>}<.>{<DT><NN>}<VBD><.>
  
  ``ChunkString`` are created from tagged texts (i.e., lists of
  ``tokens`` whose type is ``TaggedType``).  Initially, nothing is
  chunked.
  
  The chunking of a ``ChunkString`` can be modified with the ``xform()``
  method, which uses a regular expression to transform the string
  representation.  These transformations should only add and remove
  braces; they should *not* modify the sequence of angle-bracket
  delimited tags.
  
  :type _str: str
  :ivar _str: The internal string representation of the text's
      encoding.  This string representation contains a sequence of
      angle-bracket delimited tags, with chunking indicated by
      braces.  An example of this encoding is::
  
          {<DT><JJ><NN>}<VBN><IN>{<DT><NN>}<.>{<DT><NN>}<VBD><.>
  
  :type _pieces: list(tagged tokens and chunks)
  :ivar _pieces: The tagged tokens and chunks encoded by this ``ChunkString``.
  :ivar _debug: The debug level.  See the constructor docs.
  
  :cvar IN_CHUNK_PATTERN: A zero-width regexp pattern string that
      will only match positions that are in chunks.
  :cvar IN_STRIP_PATTERN: A zero-width regexp pattern string that
      will only match positions that are in strips.
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code: def _tag(self, tok):
      if isinstance(tok, tuple):
          return tok[1]
      elif isinstance(tok, Tree):
          return tok.label()
      else:
          raise ValueError(
              "chunk structures must contain tagged " "tokens or trees"
          )
  
imports: import re
  import regex
  from nltk.chunk.api import ChunkParserI
  from nltk.tree import Tree
label_desc: Use image node_id 2 for calling the ChunkString obj's underlying member method code with example usage: obj._tag(tok) and returns: tok, tok