Question about tcl command 'set' in 'proc'

[=?EUC-KR?B?s+vBpMjG?= <OHNOHOYA@chollian.net>]

I have a problem with the following Tcl code.It's for NS-1.4,
and the interpreter gave a message,
" can't read "ssm:" no such variable "

proc sarmoa { command index layer node } {
	if { $command == "set-sender" } {
		set ssm$index [ns agent sarmoa_sender_manager $node]

		set ssc [expr $index*100]
		for { set i 0 } { $i < $layer } { incr i } {
			set ss$ssc [ns agent sarmoa_sender $node]
			$ssm$index layer-port $i [$ss$ssc port]
			$ss$ssc manager-port [$ssm$index port]
			incr ssc
		}
	}
}

But the set command ,
>> set ssm$index [ns agent sarmoa_sender_manager $node]
worked well in the ns prompt. It made me crazy when included in
the proc command.
I don't understand why the set command generates errors.

Any suggestion would be great help to me!
Thank you in advance.