Some definitions of interest.
band	Def pq == if p q else false fi
	Thm* p,q:. (pq)
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

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