the_w:World, e:E. isnull(act(e)) 

latex

Definitions	Action(i), loc(e), A, b, isnull(a), act(e), E, World, x:A. B(x), t  T, P  Q, False
Lemmas	w-isnull wf, assert wf, not wf, w-loc wf, w-action wf, w-act wf, world wf, w-E wf