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
map	Def map(f;as) == Case of as; nil  nil ; a.as'  f(a).map(f;as')  (recursive)
	Thm* A,B:Type, f:(AB), l:A List. map(f;l)  B List

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