Find all the nodes where there is an edge from the specified node to that node.
        Returns a list of node ids.

def neighbors(self, node_id):
        """Find all the nodes where there is an edge from the specified node to that node.
        Returns a list of node ids."""
        node = self.get_node(node_id)
        return [self.get_edge(edge_id)['vertices'][1] for edge_id in node['edges']]