[Zope] Calling a DTML Document using a string

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