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
upto	Def upto(n) == if n=0 nil else upto(n-1) @ [(n-1)] fi  (recursive)
	Thm* n:. upto(n)  n List

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