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
compose_iter	Def  f{i}(x) == if i=0 x else f(f{i-1}(x)) fi  (recursive)
	Thm*  f:(AA), i:. f{i}  AA
nat	Def   == {i:| 0i }
	Thm*    Type

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