code: public static NaryTreebank readTreesInPtbFormat(Reader reader) throws IOException {
          NaryTreebank trees = new NaryTreebank();
          while (true) {
              NaryTree tree = NaryTree.readTreeInPtbFormat(reader);
              if (tree != null) {
                  tree.intern();
                  trees.add(tree);
              }
              if (tree == null) {
                  break;
              }
          }
          return trees;
      }
docs: Reads a list of trees in Penn Treebank format.