Some definitions of interest.
biject	Def  Bij(A; B; f) == Inj(A; B; f) & Surj(A; B; f)
	Thm*  A,B:Type, f:(AB). Bij(A; B; f)  Prop
compose_iter	Def  f{i}(x) == if i=0 x else f(f{i-1}(x)) fi  (recursive)
	Thm*  f:(AA), i:. f{i}  AA
nat	Def   == {i:| 0i }
	Thm*    Type

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