docs: @Override
    public void apply(TreeVisitor tv) {
      for (Tree t : tb) {
        if (VERBOSE) System.out.println("TfTbApply transforming " + t);
        Tree tmpT = t.deepCopy();
        if (transformer != null) {
          tmpT = transformer.transformTree(tmpT);
        }
        if (VERBOSE) System.out.println("  to " + tmpT);
        tv.visitTree(tmpT);
      }
    }