A Weak Topological Order is a permutation of the vertices of a graph such that for every edge u→v, either
- Forward Edge: u precedes v
- Backward Edge: v precedes or equals u, and v is the head of a component.
通常拓扑排序只有有向无环图才能实现，而wto可以用于CFG中有循环时计算不动点。
wto也可以用括号表示自然循环/component：
A hierarchical ordering of a set is a well-parenthesized permutation of its elements without two consecutive ”(“. I defines a total order ⇐ over its elements. The elements between two matching parentheses are called a Component. The first element of a Component is called the head. Let denote by H(v) the set of head of the nested components containing v.
例如1(234)5 表示234构成一个component