Some definitions of interest.
compose	Def  (f o g)(x) == f(g(x))
	Thm*  A,B,C:Type, f:(BC), g:(AB). f o g  AC
	Thm*  A,B,C:Type, f:(B inj C), g:(A inj B). f o g  A inj C
	Thm*  f:(B onto C), g:(A onto B). f o g  A onto C
inv_pair	Def  AB == {fg:((AB)(BA))| fg/f,g. InvFuns(A;B;f;g) }
	Thm*  A,B:Type. AB  Type

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