Def  !A == {x:A| y:A. x = y }

is mentioned by

Thm*  !{p:()| x:. p(x)  prime(x) }

[prime_decider_exists]

In prior sections: LogicSupplement

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