def reverse_each(&block)
      node = @last_leaf
      while node
        break unless node.reverse_each(&block)
        node = node.prev_sibling
      end
    end
def reverse_each(&block)
      node = @last_leaf
      while node
        break unless node.reverse_each(&block)
        node = node.prev_sibling
      end
    end
Iterate over all entries in the tree in reverse order. Entries are
 always sorted by the key.
 @yield [key, value]