[Zope] Typecasting in DTML (or is this Python?)

Hello again list... I need to typecast [amount] as a string - but only within the history string. in my attempt below, history finished up as.... Advertising authority number 25020030718 For $['amount'] Where I need amount to show up as the specific amount. <dtml-call "REQUEST.set('amount',_['amount'])"> <dtml-call "REQUEST.set('history',_['history1'] + REQUEST['authority'] + ' For $' + _.str(['amount']))"> I've tried a few variants, but not had much joy. Any help would be fantastic... thanks in advance Julian