[Zope] replace values in a list or dictionary

Hi, How can I replace the value in a list defined in REQUEST? For example, in a DTML Document: <!--#call "REQUEST.set('L',['a',2,3,4,5])"--> How can I replace the value 'a' in list L with 1 so that L become [1,2,3,4,5]? In Python, it's easy. L[0]=1. How can one do the same thing in Zope DTML Document? T.C. Huang