[Zope] global varible to files

[Jianping Zhu]

I in a file i have code like this:
       <dtml-if "teach=='y'">
         <dtml-let t="'Yes'"></dtml-let>
	 <dtml-else>
       <dtml-let t="'No'"></dtml-let>
        </dtml-if>

I want to access varible t outside this if loop how can i declear it
global to the entire file?

Thanks