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

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