add precedence relations to the document graph (i.e. an edge from the
        root node to the first token node, an edge from the first token node to
        the second one etc.)

def add_precedence_relations(self):
        """
        add precedence relations to the document graph (i.e. an edge from the
        root node to the first token node, an edge from the first token node to
        the second one etc.)
        """
        assert len(self.tokens) > 1, \
            "There are no tokens to add precedence relations to."
        self.add_edge(self.root, self.tokens[0],
                      layers={self.ns, self.ns+':precedence'},
                      edge_type=EdgeTypes.precedence_relation)
        for i, token_node_id in enumerate(self.tokens[1:]):
            # edge from token_n to token_n+1
            self.add_edge(self.tokens[i], token_node_id,
                          layers={self.ns, self.ns+':precedence'},
                          edge_type=EdgeTypes.precedence_relation)