Some definitions of interest.
assert	Def  b == if b True else False fi
	Thm*  b:. b  Prop
exteq	Def  A =ext B == (x:A. x  B) & (x:B. x  A)

About: IF YOU CAN SEE THIS go to /sfa/Nuprl/Shared/Xindentation_hack_doc.html