Ruby Hash#only, Hash#except

I was wondering the other day if the built-in Ruby Hash class had a way of applying a blacklist or a whitelist to filter out certain keys. I couldn't find anything, so I rolled up a little thing of my own, in case you're interested:

class Hash
  def except(*blacklist)
    {}.tap do |h|
      (keys - blacklist).each { |k| h[k] = self[k] }
    end
  end

  def only(*whitelist)
    {}.tap do |h|
      (keys & whitelist).each { |k| h[k] = self[k] }
    end
  end
end

Now you can do things like this:

h = {:a => 10, :b => 34, :c => "hello"}
h.only :a, :b     #=> {:a => 10, :b => 34}
h.except :a, :b   #=> {:c => "hello"}

Note that the code only works with Ruby 1.9 or higher, or with the andand gem installed because it depends on Object#tap.

If anybody has suggestions/improvements, or knows that this has been done already and can point me to it, feel free to comment.