Some definitions of interest.
w-E	Def E == {p:(Id)| isnull(a(1of(p);2of(p))) }
world	Def World Def == T:IdIdType Def == TA:IdIdType Def == M:IdLnkIdType Def == (i:Id(x:IdT(i,x)))(i:Idaction(w-action-dec(TA;M;i))) Def == (i:Id({m:Msg(M)| source(mlnk(m)) = i } List))Top
	Thm* World  Type{i'}
Id	Def Id == Atom
	Thm* Id  Type
nat	Def  == {i:| 0i }
	Thm*   Type
w-first	Def first(e) Def == if time(e)=0 true Def == i; isnull(a(loc(e);time(e)-1)) first(<loc(e),time(e)-1>) Def == else false fi Def (recursive)

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