code: @Override
    public List<TaggedWord> tag(String word) {
      List<TaggedWord> lookedUpTerms = mapping.get(word);
      if (lookedUpTerms != null) {
        return Collections.unmodifiableList(lookedUpTerms);
      } else {
        return Collections.emptyList();
      }
    }
docs: Look up a word's baseform (lemma) and POS information.