expression -- XPath compiled expression

def evaluate(self, expression, processorNss=None):
        '''expression -- XPath compiled expression
        '''
        from Ft.Xml import XPath
        if not processorNss:
            context = XPath.Context.Context(self.node, processorNss=self.processorNss)
        else:
            context = XPath.Context.Context(self.node, processorNss=processorNss)
        nodes = expression.evaluate(context)
        return map(lambda node: ElementProxy(self.sw,node), nodes)