Some definitions of interest.
le	Def AB == B<A
	Thm* i,j:. (ij)  Prop
length	Def ||as|| == Case of as; nil  0 ; a.as'  ||as'||+1  (recursive)
	Thm* A:Type, l:A List. ||l||
	Thm* ||nil||
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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