Who Cites filter?
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
reduce	Def reduce(f;k;as) == Case of as; nil  k ; a.as'  f(a,reduce(f;k;as')) Def (recursive)
	Thm* A,B:Type, f:(ABB), k:B, as:A List. reduce(f;k;as)  B

Syntax:

filter(P;l)

has structure:

filter(P; l)

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