code: def _do_relation(self):
          
          if self.chunks:
              ch = self.chunks[-1]
              for relation, role in ch.relations:
                  if role == "SBJ" or role == "OBJ":
                      self.relations[role][relation] = ch
              if ch.type in ("VP",):
                  self.relations[ch.type][ch.relation] = ch
docs: Attaches subjects, objects and verbs.
              If the previous chunk is a subject/object/verb, it is stored in Sentence.relations{}.