Apply a function on given word

        :param str function_name: function to apply
        :param str word: word to apply function on
        :rtype: Query

def function(self, function_name, word):
        """ Apply a function on given word

        :param str function_name: function to apply
        :param str word: word to apply function on
        :rtype: Query
        """
        word = self._parse_filter_word(word)

        self._add_filter(
            *self._prepare_function(function_name, self._attribute, word,
                                    self._negation))
        return self