'And an extra filter (to be applied on pre-filtered subsets)
@param method: a name of a known filter method, or a
callable filter method
@param expression: the filter expression (string)'

def add_extra_filter(self, method, expression):

    self.clear()
    if (self.rfilter is None):
        self.rfilter = S3ResourceFilter(self)
    self.rfilter.add_extra_filter(method, expression)
