docs: def attribute_checker(operator, attribute, value=''):
      """
      Takes an operator, attribute and optional value; returns a function that
      will return True for elements that match that combination.
      """
      return {
          '=': lambda el: el.get(attribute) == value,
          # attribute includes value as one of a set of space separated tokens
          '~': lambda el: value in el.get(attribute, '').split(),
          # attribute starts with value
          '^': lambda el: el.get(attribute, '').startswith(value),
          # attribute ends with value
          '$': lambda el: el.get(attribute, '').endswith(value),
          # attribute contains value
          '*': lambda el: value in el.get(attribute, ''),
          # attribute is either exactly value or starts with value-
          '|': lambda el: el.get(attribute, '') == value \
              or el.get(attribute, '').startswith('%s-' % value),
      }.get(operator, lambda el: el.has_key(attribute))