(
x
L1.(
a.reduce(b,y. 
msg(a,b)
y;true;L2))(x))
A
a,b:A. (a L1) 
(b L2) msg(a,b) filter(a.reduce(b,y. msg(a,b)y;true;L2);L1) = L1P:(T), L:T List. (xL.P(x)) filter(P;L) = L| 1 | (xL1.(a.reduce(b,y. msg(a,b)y;true;L2))(x)) |
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