w-pred(w;e)
== if time(e)=0 inr()
== i; isnull(a(loc(e);time(e)-1)) w-pred(w;<loc(e),time(e)-1>)
== else inl(<loc(e),time(e)-1>) fi
(recursive) 

latex

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clarification:

w-pred(w;e)
== if w-time(w; e)=0 inr()
== i; w-isnull(w; w-a(w; w-loc(w; e); (w-time(w; e)-1))) w-pred(w;<w-loc(w; e),w-time(w; e)-1>)
== else inl(<w-loc(w; e),w-time(w; e)-1>) fi
(recursive) 

latex

Definitions	Y, x.A(x), i=j, inr(x), , if b t else f fi, isnull(a), a(i;t), f(a), inl(x), <a,b>, loc(e), n-m, time(e), #$n
FDL editor aliases	w-pred