>>> import treebank_grammar as tg
>>> g = tg.extract_simple_pcfg(1000)
>>> tg.print_grammar(g, "PP")
PP -> ADJ ADP NP [0.00413394]
PP -> ADJ ADV PP PUNCT ADV PP [0.000413394]
PP -> ADJ NP [0.000413394]
PP -> ADP ADJP [0.000413394]
PP -> ADP ADP NP [0.00289376]
PP -> ADP ADVP [0.00289376]
PP -> ADP FRAG [0.000413394]
PP -> ADP NP [0.827615]
PP -> ADP NP ADVP [0.000413394]
PP -> ADP NP NP [0.000413394]
PP -> ADP NP PP [0.00124018]
...
