code: def add_precedence_relations(self):
          
          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)
docs: 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.)