def add_arc(self, head_address, mod_address):
        relation = self.nodes[mod_address]['rel']
        self.nodes[head_address]['deps'].setdefault(relation, [])
        self.nodes[head_address]['deps'][relation].append(mod_address)
        #self.nodes[head_address]['deps'].append(mod_address)
Adds an arc from the node specified by head_address to the
dataset/ETHPy150Open nltk/nltk/nltk/parse/dependencygraph.py/DependencyGraph.add_arc