'Construct a new ``ChunkString`` that encodes the chunking of
the text ``tagged_tokens``.
:type chunk_struct: Tree
:param chunk_struct: The chunk structure to be further chunked.
:type debug_level: int
:param debug_level: The level of debugging which should be
applied to transformations on the ``ChunkString``.  The
valid levels are:
- 0: no checks
- 1: full check on to_chunkstruct
- 2: full check on to_chunkstruct and cursory check after
each transformation.
- 3: full check on to_chunkstruct and full check after
each transformation.
We recommend you use at least level 1.  You should
probably use level 3 if you use any non-standard
subclasses of ``RegexpChunkRule``.'

def __init__(self, chunk_struct, debug_level=1):

    self._root_label = chunk_struct.label()
    self._pieces = chunk_struct[:]
    tags = [self._tag(tok) for tok in self._pieces]
    self._str = ((u'<' + u'><'.join(tags)) + u'>')
    self._debug = debug_level
