code: def get(self, term):
          # type: (Any) -> Type[ChomskyTermNonterminal]
          
          if self._items[term].used is False:
              cont = self._items[term]
              self._grammar.nonterminals.add(cont.nonterminal)
              self._grammar.rules.add(cont.rule)
              cont.used = True
          return self._items[term].nonterminal
docs: Get nonterminal rewritable to term.
          If the rules is not in the grammar, nonterminal and rule rewritable to terminal are add into grammar.
          :param term: Term for which get the nonterminal.
          :return: ChomskyTermNonterminal class for terminal.