Some definitions of interest.
biject	Def Bij(A; B; f) == Inj(A; B; f) & Surj(A; B; f)
	Thm* A,B:Type, f:(AB). Bij(A; B; f)  Prop
compose	Def (f o g)(x) == f(g(x))
	Thm* A,B,C:Type, f:(BC), g:(AB). f o g  AC
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
tidentity	Def Id == Id
	Thm* A:Type. Id  AA

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