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||
nat	Def  == {i:| 0i }
	Thm*   Type
select	Def l[i] == hd(nth_tl(i;l))
	Thm* A:Type, l:A List, n:. 0n  n<||l||  l[n]  A
zip	Def zip(as;bs) Def == Case of as Def == Canil  nil Def == Caa.as'  Case of bs; nil  nil ; b.bs'  [<a,b> / zip(as';bs')] Def (recursive)
	Thm* T1,T2:Type, as:T1 List, bs:T2 List. zip(as;bs)  (T1T2) List

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