Some definitions of interest.
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
surject	Def Surj(A; B; f) == b:B. a:A. f(a) = b
	Thm* A,B:Type, f:(AB). Surj(A; B; f)  Prop

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