[ns] i'm very new with ns

[Raymond Wang <rayw@stanford.edu>]

hi,

i'm very new with ns.  here is a snippet of my code (actually borrowed from 
someone with permission and i'm trying to understand it), why does it 
result in the errors below?

thanks a lot!
ray

proc record {} {
         global sink0 f0
         #Get an instance of the simulator
         set ns [Simulator instance]
         #Set the time after which the procedure should be called again
         set time 0.5
         #How many bytes have been received by the traffic sinks?
         set bw0 [$sink0 set bytes_]
         #Get the current time
         set now [$ns now]
         #Calculate the bandwidth (in MBit/s) and write it to the files
         puts $f0 "$now [expr $bw0/$time*8/1000000]"
         #Reset the bytes_ values on the traffic sinks
         $sink0 set bytes_ 0
         #Re-schedule the procedure
         $ns at [expr $now+$time] "record"
}

Errors:
ns: record: can't read "bytes_": no such variable
     while executing
"subst $[subst $var"
     (procedure "_o44" line 5)
     (Object next line 5)
     invoked from within
"_o44 next bytes_"
     ("eval" body line 1)
     invoked from within
"eval $self next $args"
     (procedure "_o44" line 18)
     (Agent set line 18)
     invoked from within
"$sink0 set bytes_"
     (procedure "record" line 8)
     invoked from within
"record"