Update dictionary from a collection of documents. Each document is a list
        of tokens.

        Args:
            docs (list): documents to add.

def add_documents(self, docs):
        """Update dictionary from a collection of documents. Each document is a list
        of tokens.

        Args:
            docs (list): documents to add.
        """
        for sent in docs:
            sent = map(self.process_token, sent)
            self._token_count.update(sent)