If a variable contains a nodeset from a page source and the nodeset is modified during processing, then the variable is invalidated and cannot be used subsequently. There are two situations in which alternative processing is desirable:
•Changes to a nodeset occur via a sequence of multiple actions, each of which changes the nodeset. This leads to a situation in which each action is processing a nodeset that has been modified by a previous action.
•The evaluation of an XPath expression containing a variable that references a modified page source (for example, if the XPath expression occurs within a Loop action).
The Execute at Once action does the following:
1.Evaluates the XPath/XQuery statements of all its child actions
2.Applies the effects of these evaluations to the relevant nodesets of the page.