问题 ['', ['S', ['NP-SBJ', ['NP', ['NNP', 'Pierre'], ['NNP', 'Vinken']], [',', ','], ['ADJP', ['NP', ['CD', '61'], ['NNS', 'years']], ['JJ', 'old']], [',', ',']], ['VP', ['MD', 'will'], ['VP', ['VB', 'join'], ['NP', ['DT', 'the'], ['NN', 'board']], ['PP-CLR', ['IN', 'as'], ['NP', ['DT', 'a'], ['JJ', 'nonexecutive'], ['NN', 'director']]], ['NP-TMP', ['NNP', 'Nov.'], ['CD', '29']]]], ['.', '.']]] I want to generate production rules by traversing this grammar using NLTK Tree. Production rules would