It is activated before the document is presented to the user. This is also a reflection of Nelson's concept of 'stretch text'. There are 3 further types which I believe are necessary to complete the function paradigm. The point that was mentioned about the lack of an ubiquitious scripting language is well made. Lisp is too arcane for most.
Shell languages are too platform specific.
What is needed is a simple to understand, freely available scripting platform. This message creates a new link in the 'user document' of the individual who placed the attention. In this way, I could place a link onto a document I had interest in, and when it was changed or accessed in some manner, I would be informed. This is the concept of a 'knowbot'. Collection links can be activated based on day and time, much like the WAIS questions in the MAC WAIS interface, WAIS-Station.