    def stem(self, input_string):
        """Apply stemming."""
        word_and_pos = nltk.pos_tag(nltk.tokenize.word_tokenize(input_string))
        stemmed_words = []
        for w, p in word_and_pos:
            if w in _MANUAL_MATCHES:
                w = _MANUAL_MATCHES[w]
            elif w.endswith("ing"):
                w = self._wordnet_lemmatizer.lemmatize(w, VERB)
            elif p.startswith("NNS") or p.startswith("NNPS"):
                w = inflection.singularize(w)
            stemmed_words.append(w)
        return " ".join(stemmed_words)
Apply stemming.
open_flamingo/eval/ok_vqa_utils.py