Some definitions of interest.
nat	Def   == {i:| 0i }
	Thm*    Type
nat_plus	Def   == {i:| 0<i }
	Thm*    Type
not	Def  A == A  False
	Thm*  A:Prop. (A)  Prop

About: IF YOU CAN SEE THIS go to http://www.cs.cornell.edu/Info/People/sfa/Nuprl/Shared/Xindentation_hack_doc.html