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
one_one_corr_2	Def  A ~ B == f:(AB), g:(BA). InvFuns(A;B;f;g)
	Thm*  A,B:Type. (A ~ B)  Prop
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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