code: @Override
    public Treebank transform(TreeTransformer treeTrans) {
      Treebank mtb = new MemoryTreebank(size(), treeReaderFactory());
      for (Tree t : this) {
        mtb.add(treeTrans.transformTree(t));
      }
      return mtb;
    }
docs: Return a MemoryTreebank where each
  Tree in the current treebank has been transformed using the
  TreeTransformer.  This Treebank is unchanged (assuming that the
  TreeTransformer correctly doesn't change input Trees).
  
  @param treeTrans The TreeTransformer to use