 tr':|E| List. tr'
tr':|E| List. tr'  x
 x 
 (
 ( x
x tr'.(
tr'.( y
y tr'.is-send(E)(y)  &  (y =msg=(E) x)))
tr'.is-send(E)(y)  &  (y =msg=(E) x))) b.
b.
 (b =msg=(E) a);x)
(b =msg=(E) a);x) y
 y (
 ( x
x tr'.(
tr'.( y
y tr'.is-send(E)(y)  &  (y =msg=(E) x)))
tr'.is-send(E)(y)  &  (y =msg=(E) x))) tr:|E| List. tr
tr:|E| List. tr  x  &  tr' = filter(
 x  &  tr' = filter( b.
b.
 (b =msg=(E) a);tr))
(b =msg=(E) a);tr))| 1 |    tr:|E| List. tr  x  &  tr' = filter(  b.   (b =msg=(E) a);tr) | 
| 2 | 9.  tr:|E| List. tr  x  &  tr' = filter(  b.   (b =msg=(E) a);tr)  (  x  tr'.(  y  tr'.is-send(E)(y)  &  (y =msg=(E) x))) | 
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