Some definitions of interest.
inv_pair	Def  AB == {fg:((AB)(BA))| fg/f,g. InvFuns(A;B;f;g) }
	Thm*  A,B:Type. AB  Type
one_one_corr_2	Def  A ~ B == f:(AB), g:(BA). InvFuns(A;B;f;g)
	Thm*  A,B:Type. (A ~ B)  Prop

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