步驟運算必須包含節點測試，如果是使用述詞，則必須包含用以對前一個節點集識別之節點集中的每個節點，進行測試的代數運算式，如此才能進行評估。

A step operation must involve either a node test or, in the case of a predicate, an algebraic expression against which to test each node in the node set identified by the preceeding node set can be evaluated.