[Zope] Calling a DTML Document using a string

[Anderson Ami]

How can I do a dtml document call using the call dtml tag ?
e.g.

- I have 3 DTML documents ( bot1, bot2 and bot3)
- I have in my request a int variable called varX
- I would like do it, but this one doesn't work :
...  
<dtml-var expr="bot + varX">
...

Thanks a lot
anderson