Add conjunction to grammar rule - NLTK parse into syntax tree in python

Question

Suppose we have following incomplete grammar rule:

grammar2 = nltk.parse_cfg("""
S -> NP VP
NP -> Det N
VP -> V NP
PN -> 'David' 
Det -> 'the'
N -> 'man' 
V -> 'saw' | 'helped' 
Pro -> 'him'
Conj -> 'and'
""")

I want to create syntax tree for following sentence:

sent = ['the', 'man', 'saw', 'David', 'and', 'helped', 'him']
parser = nltk.ChartParser(grammar2)
trees = parser.nbest_parse(sent)
for tree in trees:
    print tree

But I don't know how can I add conjunction and in the grammar?

Answer 1

A job for recursion. This should work.

S -> NP VP 
VP -> V NP | V NP Conj VP
NP -> Det N | PN | Pro
PN -> 'David' 
Det -> 'the'
N -> 'man' 
V -> 'saw' | 'helped' 
Pro -> 'him'
Conj -> 'and'