[Zope] global varible to files

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