Some definitions of interest.
inv_funs_2	Def  InvFuns(A;B;f;g) == (x:A. g(f(x)) = x) & (y:B. f(g(y)) = y)
	Thm*  f:(AB), g:(BA). InvFuns(A;B;f;g)  Prop
singleton	Def  {a:T} == {x:T| x = a  T }
	Thm*  T:Type, a:T. {a:T}  Type

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