def to_tree_s
      str = ''

      each do |node, mode, stack|
        if mode == :on_enter
          begin
            branch_mark = node.parent.nil? ? '' :
              node.parent.smaller == node ? '<' :
              node.parent.equal == node ? '=' :
              node.parent.larger == node ? '>' : '@'

            str += "#{node.text_tree_prefix}#{branch_mark}-" +
              "#{node.smaller || node.equal || node.larger ? 'v-' : '--'}" +
              "#{node.to_s}\n"
          rescue
            str += "#{node.text_tree_prefix}- @@@@@@@@@@\n"
          end
        end
      end

      str
    end
def to_tree_s
      str = ''

      each do |node, mode, stack|
        if mode == :on_enter
          begin
            branch_mark = node.parent.nil? ? '' :
              node.parent.smaller == node ? '<' :
              node.parent.equal == node ? '=' :
              node.parent.larger == node ? '>' : '@'

            str += "#{node.text_tree_prefix}#{branch_mark}-" +
              "#{node.smaller || node.equal || node.larger ? 'v-' : '--'}" +
              "#{node.to_s}\n"
          rescue
            str += "#{node.text_tree_prefix}- @@@@@@@@@@\n"
          end
        end
      end

      str
    end
Convert the node and all child nodes into a tree like text form.
 @return [String]