Who Cites find?
find	Def (first x as s.t. P(x) else d) == Case of filter(x.P(x);as); nil d ; a.b a
	Thm* T:Type, P:(T), as:T List, d:T. (first a as s.t. P(a) else d) T
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')) (recursive)
	Thm* A,B:Type, f:(ABB), k:B, as:A List. reduce(f;k;as) B

Syntax:

(first x as s.t. P(x) else d)

has structure:

find(x.P(x); as; d)

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