RAA - orderedhash/1.2005.8.16

# You can simply do
  hsh = OrderedHash.new
  hsh['z'] = 1
  hsh['a'] = 2
  hsh['c'] = 3
  p hsh.keys     # ['z','a','c']

# or using OrderedHash[] method
  hsh = OrderedHash['z', 1, 'a', 2, 'c', 3]
  p hsh.keys     # ['z','a','c']

# OrderedHash has useful extensions: push, pop and unshift
  p hsh.push('to_end', 15)  # true, key added
  p hsh.push('to_end', 30)  # false, already - nothing happen
  p hsh.unshift('to_begin', 50)  # true, key added
  p hsh.unshift('to_begin', 60)  # false, already - nothing happen
  p hsh.keys     # ["to_begin", "a", "c", "z", "to_end"]
  p hsh.pop      # ["to_end", 15], if nothing remains, return nil
  p hsh.keys     # ["to_begin", "a", "c", "z"]
  p hsh.shift    # ["to_begin", 30], if nothing remains, return nil