class Hash
  # Returns a new hash created by traversing the hash and its subhashes,
  # executing the given block on the key and value. The block should 
  # return a 2-element array of the form +[key, value]+.
  #
  #   require 'facet/hash/traverse'
  #
  #   h = { "A"=>"A", "B"=>"B" }
  #   h.traverse { |k,v| [k.downcase, v] }
  #   h  #=> { "a"=>"A", "b"=>"B" }
  #
  def traverse(&b)
    inject({}) do |h,(k,v)|
      nk, nv = b[k,v]
      h[nk] = ( Hash === v ? v.traverse(&b) : nv )
      h
    end
  end
end