code: public int[] getSentenceIds(IntObjectBimap<String> lexAlphabet) {
          ArrayList<NaryTree> leaves = getLexicalLeaves();
          int[] sent = new int[leaves.size()];
          for (int i=0; i<sent.length; i++) {
              sent[i] = lexAlphabet.lookupIndex(leaves.get(i).symbol);
          }
          return sent;
      }
docs: Gets the lexical item ids comprising the sentence.