Some definitions of interest.
inject	Def  Inj(A; B; f) == a1,a2:A. f(a1) = f(a2)  B  a1 = a2
	Thm*  A,B:Type, f:(AB). Inj(A; B; f)  Prop
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

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