Some definitions of interest.
inhabited_uniquely	Def  !A == {x:A| y:A. x = y }
injection_type	Def  A inj B == {f:(AB)| Inj(A; B; f) }
	Thm*  A,B:Type. A inj B  Type
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
not	Def  A == A  False
	Thm*  A:Prop. (A)  Prop

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