class_docstr: Object for core-components (POS, Dependency Relations, etc).
  
  Attributes:
      _doc_text: the doc text
      _sentences: list of sentences, each word in a sentence is
          represented by a dictionary, structured as follows: {'start': (int), 'len': (int),
          'pos': (str), 'ner': (str), 'lemma': (str), 'gov': (int), 'rel': (str)}
code: def __len__(self):
      return len(self.sentences)
  
imports: import json
  from nlp_architect.utils.io import validate
label_desc: Use image node_id 10 for calling the CoreNLPDoc obj's underlying member method code with example usage: obj.__len__() and returns: len