Expect with different passwords

proc login {user pass} {
    expect "login:"
    send "$user\r"
    expect "password:"
    send "$pass\r"
}
set username spongebob set passwords {squarepants rhombuspants}
set index 0

spawn telnet 192.168.40.100
login $username [lindex $passwords $index]
expect {
    "login incorrect" {
        send_user "failed with $username:[lindex $passwords $index]\n"
        incr index
        if {$index == [llength $passwords]} {
            error "ran out of possible passwords"
        }
        login $username [lindex $passwords $index]
        exp_continue
    }
    "prompt>" }
send_user "success!\n"
# ...
 

http://stackoverflow.com/questions/1538444/using-conditional-statements-inside-expect