mb event system 3 Sections EventSystems Doc
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Def w.TA == 1of(2of(w))

is mentioned by

Def valtype(i;a) == kindcase(kind(a);a.w.TA(i,a);l,tg.w.M(l,tg))[w-valtype]
Def Action(i) == action(w-action-dec(w.TA;w.M;i))[w-action]

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

mb event system 3 Sections EventSystems Doc