Some definitions of interest.
length	Def ||as|| == Case of as; nil  0 ; a.as'  ||as'||+1  (recursive)
	Thm* A:Type, l:A List. ||l||
	Thm* ||nil||
select	Def l[i] == hd(nth_tl(i;l))
	Thm* A:Type, l:A List, n:. 0n  n<||l||  l[n]  A

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