acyclic directed graph的用法和样例：
- A multiobjective shortest path evolutionary algorithm is presented for acyclic directed graph.
- A belief network is a probability model defined on an acyclic directed graph; distributed means nodes can be on different hosts, and heterogeneous means allowing different conditional distributions.
- A belief network is a probability model defined on an acyclic directed graph;
- We shall abbreviate "directed graph" to digraph.