def each(value = nil, &block)
      yield self, value
      if @children
        @children.each do |child|
          child.each value, &block
        end
      end
      value
    end
def each(value = nil, &block)
      yield self, value
      if @children
        @children.each do |child|
          child.each value, &block
        end
      end
      value
    end
Process each node beginning with the current node.