code: public static IntNaryTreebank readTreesInPtbFormat(IntObjectBimap<String> lexAlphabet, IntObjectBimap<String> ntAlphabet, Reader reader) throws IOException {
          IntNaryTreebank trees = new IntNaryTreebank();
          while (true) {
              IntNaryTree tree = IntNaryTree.readTreeInPtbFormat(lexAlphabet, ntAlphabet, reader);
              if (tree != null) {
                  trees.add(tree);
              }
              if (tree == null) {
                  break;
              }
          }
          return trees;
      }
docs: Reads a list of trees in Penn Treebank format.