property() but encapsulates it in a list, if it's a
        single-element property.

def property_as_list(self, property_name):
        """ property() but encapsulates it in a list, if it's a
        single-element property.
        """
        try:
            res = self._a_tags[property_name]
        except KeyError:
            return []

        if type(res) == list:
            return res
        else:
            return [res]