Some definitions of interest.
filter	Def filter(P;l) == reduce(a,v. if P(a) [a / v] else v fi;nil;l)
	Thm* T:Type, P:(T), l:T List. filter(P;l)  T List
iseg	Def l1  l2 == l:T List. l2 = (l1 @ l)
	Thm* T:Type, l1,l2:T List. l1  l2  Prop

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