Some definitions of interest.
nat	Def   == {i:| 0i }
	Thm*    Type
nat_to_nat_pair	Def  nat_to_nat_pair(i) == next_nat_pair{i}(<0,0>)
	Thm*  nat_to_nat_pair
surject	Def  Surj(A; B; f) == b:B. a:A. f(a) = b
	Thm*  A,B:Type, f:(AB). Surj(A; B; f)  Prop

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