'Adds an arc from the node specified by head_address to the
node specified by the mod address.'

def add_arc(self, head_address, mod_address):

    relation = self.nodes[mod_address][u'rel']
    self.nodes[head_address][u'deps'].setdefault(relation, [])
    self.nodes[head_address][u'deps'][relation].append(mod_address)
