is-send(E)(x[(i+1)]) is-send(E)(x[i]) 
loc(E)(x[i]) = loc(E)(x[(i+1)])
(||x||-1)R_ad_normal(tr)(x[i],x[(i+1)]) switch_inv(E)(swap(x;i;i+1))
E:TaggedEventStruct, x:|E| List, i:(||x||-1).
switch_inv(E)(x) 
is-send(E)(x[(i+1)])
is-send(E)(x[i]) loc(E)(x[i]) = loc(E)(x[(i+1)]) switch_inv(E)(swap(x;i;i+1))
[E;x;i]| 1 | is-send(E)(x[(i+1)]) |
| 2 | is-send(E)(x[i]) loc(E)(x[i]) = loc(E)(x[(i+1)]) |
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