Hashes of Hashes Idiom in Ruby?

h = Hash.new
h['x'] = Hash.new if not h.key?('x')
h['x']['y'] = value_to_insert

h = Hash.new
h['x']['y'] = value_to_insert

looked_up_value = h['w']['z']

h = Hash.new { |h, k| h[k] = Hash.new }

hash = Hash.new(&(p=lambda{|h,k| h[k] = Hash.new(&p)}))

((h ||= { })['w'] ||= { })['z']