is mentioned by

Def f  g == x:A. x  dom(f)  x  dom(g) & f(x) = g(x)  B(x)	[fpf-sub]
Def z != f(x) ==> P(a;z) == x  dom(f)  P(x;f(x))	[fpf-val]
Def f(x)?z == if x  dom(f) f(x) else z fi	[fpf-cap]

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