 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 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) (
 ( x
x tr'.(
tr'.( y
y tr'.is-send(E)(y)  &  (y =msg=(E) x)))
tr'.is-send(E)(y)  &  (y =msg=(E) x)))| 1 | 9. tr: |E| List 10. tr  x 11. tr' = filter(  b.   (b =msg=(E) a);tr) 12.  x:|E|. (x  tr)   (  y  tr.is-send(E)(y)  &  (y =msg=(E) x)) 13. x1: |E| 14. (x1  tr')  (x1  tr) | 
| 2 | 9. tr: |E| List 10. tr  x 11. tr' = filter(  b.   (b =msg=(E) a);tr) 12.  x:|E|. (x  tr)   (  y  tr.is-send(E)(y)  &  (y =msg=(E) x)) 13. x1: |E| 14. (x1  tr') 15. (  y  tr.is-send(E)(y)  &  (y =msg=(E) x1))  (  y  tr'.is-send(E)(y)  &  (y =msg=(E) x1)) | 
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