Some definitions of interest.
int_iseg	Def {i...j} == {k:| ik & kj }
	Thm* i,j:. {i...j}  Type
length	Def ||as|| == Case of as; nil  0 ; a.as'  ||as'||+1  (recursive)
	Thm* A:Type, l:A List. ||l||
	Thm* ||nil||
segment	Def as[m..n] == firstn(n-m;nth_tl(m;as))
	Thm* T:Type, as:T List, m,n:. (as[m..n])  T List

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