带方向的边叫有向边（ directed edge），而不带方向的边叫无向边 （undirected edge）。对无向边来说，(i, j) 和(j, i) 是一样的；而对有向边来说，它们是不同的。
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图的节点是资源和文字，节点之间用有向边连接，边上用谓词标记。
The nodes of the graph are resources and literals, and they are connected by directed edges, which are labeled with predicates.
所有的活动都用箭头链接在一起，称为有向边，它指出从一个活动到下一活动的流向。
All of the activities are linked together using arrows, called directed edges, that indicate the flow from one activity to the next.
请注意，如果并非所有的道路都是单行道，那么我们可以在图中为每一条双行道画两条有向边，每条边指明一个方向。
Note that if not all of the streets are one-way, then we can obtain a digraph by drawing for each two-way road two directed edges, one in each direction.
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