code: def train(self, nerdocs, mode_filename):
          
  
          trainer = pycrfsuite.Trainer(algorithm=self.algorithm,
                                       params={'c2': self.c2},
                                       verbose=self.verbose)
  
          for doc in nerdocs:
              for snt in doc.sentences:
                  xseq = [t.feature_list() for t in snt]
                  yseq = [t.label for t in snt]
                  trainer.append(xseq, yseq)
  
          trainer.train(mode_filename)
docs: Train a CRF model using given documents.
  
          Parameters
          ----------
          nerdocs: list of estnltk.estner.ner.Document.
              The documents for model training.
          mode_filename: str
              The fielname where to save the model.