Calling expect script inside another expect

Hi,
Am very new to expect scripting..
Can You please suggest me how to call an expect script inside another expect script..

I tried with

spawn /usr/bin/ksh 
send "expect main.exp\r"
expect $root_prompt

and

spawn /usr/bin/ksh  
send "main.exp\r"
expect $root_prompt

Both didnt work..
they are of no use..
Please help..

What are you trying to do?

Why do you need to?