Fail to reset trace variables

[yqchen <yqchen@info.sh.cn>]

Hi,

I want to begin to trace the number of queue departuring packets or
dropping packets later after simulation has run.but i fail to reset
bdepartures_ variable to zero. Can any one help me?
I use the following script sentences:

set l [ $ns link $r1 $r2]
...
$ns at $tracestart "$l set bdepartures_ 0"
$ns at $tracestart "$l set bdrops_ 0"

Thanks.

Yiqun