code: public List<String> lookup(String lemma, String posTag) {
      return mapping.get(lemma + "|" + posTag);
    }
docs: Look up a word's inflected form as specified by the lemma and POS tag.
  
  @param lemma the lemma to inflect.
  @param posTag the required POS tag.
  @return a list with all the inflected forms of the specified lemma having the specified POS tag.
  If no inflected form is found, the function returns <code>null</code>.