Tcl global variable does not seem to change in ns2

Question

While writing an ns2 simulation, I expected the variable nummsg in the Graph proc to change for every output to the file. But it remains constant at 18. I don't understand why the incr functions do not work. Any help is appreciated.

#Create scheduler
#Create an event scheduler wit multicast turned on
set ns [new Simulator -multicast on]
$ns multicast
set nummsg 18
#Turn on Tracing
set tf [open output.tr w]
$ns trace-all $tf

# Turn on nam Tracing
set fd [open mcast.nam w]
$ns namtrace-all $fd

# Create nodes
set OLDCOMM {
    set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]
set n4 [$ns node]
set n5 [$ns node]
set n6 [$ns node]
set n7 [$ns node]
}

set i 0
for {set i} {$i < 20} {incr i} {
    set n$i "[$ns node]"
}

# Create links
$ns duplex-link $n0 $n1 1.5Mb 10ms DropTail
$ns duplex-link $n0 $n2 1.5Mb 10ms DropTail
$ns duplex-link $n2 $n3 1.5Mb 10ms DropTail
$ns duplex-link $n2 $n4 1.5Mb 10ms DropTail
$ns duplex-link $n1 $n7 1.5Mb 10ms DropTail
$ns duplex-link $n1 $n5 1.5Mb 10ms DropTail
$ns duplex-link $n4 $n6 1.5Mb 10ms DropTail

#tactic
# for time
# add or delete nodes based on a rand
for {set i} {$i < 100} {incr i} {
    puts "OK $i "

}

# Routing protocol: say distance vector
#Protocols: CtrMcast, DM, ST, BST
set mproto DM
set mrthandle [$ns mrtproto $mproto {}]

# Allocate group addresses
set group1 [Node allocaddr]
set group2 [Node allocaddr]

# UDP Transport agent for the traffic source
set udp0 [new Agent/UDP]
$ns attach-agent $n0 $udp0
$udp0 set dst_addr_ $group1
$udp0 set dst_port_ 0
set cbr1 [new Application/Traffic/CBR]
$cbr1 attach-agent $udp0

# Transport agent for the traffic source
set udp1 [new Agent/UDP]
$ns attach-agent $n1 $udp1
$udp1 set dst_addr_ $group2
$udp1 set dst_port_ 0
set cbr2 [new Application/Traffic/CBR]
$cbr2 attach-agent $udp1


#$ns at 0.2 "Graph"
set g [open graph.tr w]
set g1 [open graph1.tr w]
proc Graph {} {
    global nummsg
    global ns g g1
    set time 0.5
    set now [$ns now]
    puts $nummsg
    puts $g "$now $nummsg"
    #puts $g1 "[expr rand()*8] [expr rand()*6]"
    $ns at [expr $now+$time] "Graph"
}


# Create receiver
set rcvr1 [new Agent/Null]
$ns attach-agent $n5 $rcvr1
$ns at 1.0 "$n5 join-group $rcvr1 $group1"
incr nummsg


set rcvr2 [new Agent/Null]
$ns attach-agent $n6 $rcvr2
$ns at 1.5 "$n6 join-group $rcvr2 $group1"
incr nummsg

set rcvr3 [new Agent/Null]
$ns attach-agent $n7 $rcvr3
$ns at 2.0 "$n7 join-group $rcvr3 $group1"
set nummsg 22
#$ns duplex-link $n4 $n6 1.5Mb 10ms DropTail


set rcvr4 [new Agent/Null]
$ns attach-agent $n5 $rcvr1
$ns at 2.5 "$n5 join-group $rcvr4 $group2"
incr nummsg
set rcvr5 [new Agent/Null]
$ns attach-agent $n6 $rcvr2
$ns at 3.0 "$n6 join-group $rcvr5 $group2"
incr nummsg
set rcvr6 [new Agent/Null]
$ns attach-agent $n7 $rcvr3
$ns at 3.5 "$n7 join-group $rcvr6 $group2"

$ns at 4.0 "$n5 leave-group $rcvr1 $group1"
$ns at 4.5 "$n6 leave-group $rcvr2 $group1"
$ns at 5.0 "$n7 leave-group $rcvr3 $group1"
set nummsg [expr {$nummsg - 1}]
set nummsg [expr {$nummsg - 1}]
set nummsg [expr {$nummsg - 1}]


$ns at 5.5 "$n5 leave-group $rcvr4 $group2"
$ns at 6.0 "$n6 leave-group $rcvr5 $group2"
$ns at 6.5 "$n7 leave-group $rcvr6 $group2"
set nummsg [expr {$nummsg - 1}]
set nummsg [expr {$nummsg - 1}]
set nummsg [expr {$nummsg - 1}]

# Schedule events
$ns at 0.0 "Graph"
$ns at 0.5 "$cbr1 start"
$ns at 9.5 "$cbr1 stop"

$ns at 0.5 "$cbr2 start"
$ns at 9.5 "$cbr2 stop"


#post-processing
$ns at 10.0 "finish"
proc finish {} {
   global ns tf
   $ns flush-trace
   close $tf
   #exec nam mcast.nam &
   exit 0
}

# For nam
#Colors for packets from two mcast groups
$ns color 10 red
$ns color 11 green
$ns color 30 purple
$ns color 31 green

# Manual layout: order of the link is significant!
#$ns duplex-link-op $n0 $n1 orient right
#$ns duplex-link-op $n0 $n2 orient right-up
#$ns duplex-link-op $n0 $n3 orient right-down

# Show queue on simplex link n0->n1
#$ns duplex-link-op $n2 $n3 queuePos 0.5

# Group 0 source
$udp0 set fid_ 10
$n0 color red
$n0 label "Source 1"

# Group 1 source
$udp1 set fid_ 11
$n1 color green
$n1 label "Source 2"

$n5 label "Receiver 1"
$n5 color blue
$n6 label "Receiver 2"
$n6 color blue
$n7 label "Receiver 3"
$n7 color blue 

#$n2 add-mark m0 red
#$n2 delete-mark m0"

# Animation rate
$ns set-animation-rate 3.0ms

$ns run

Answer 1

I have found a solution. Instead of doing incr nummsg, I wrote a procedure for incrementing and decrementing the variable and called them instead. It works..