is mentioned by

Thm* the_w:World, e:E. isnull(act(e))	[w-act-not-null]
Def val(e) == val(act(e))	[w-eval]
Def kind(e) == kind(act(e))	[w-ekind]

Try larger context: EventSystems IF YOU CAN SEE THIS go to /sfa/Nuprl/Shared/Xindentation_hack_doc.html