Some definitions of interest.
nat	Def   == {i:| 0i }
	Thm*    Type
next_nat_pair	Def  next_nat_pair(xy) == xy/x,y. if y=0 <0,x+1> else <x+1,y-1> fi
	Thm*  next_nat_pair

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