[Zope] sum up values of two variables

Hi there, how do one sum up the values of two variables? <dtml-let a="_.len(hello.getPhysicalPath())" b="'BASE'"> <dtml-var a> <dtml-var b> </dtml-let> In other words: the result of _.len(hello.getPhysicalPath()) and the string "BASE" should sum up to one variable, something like <dtml-let a="'BASE'+_.len(hello.getPhysicalPath())"> Thank you Marcus