http://javieracero.com/blog/the-key-to-ruby-hashes-is-eql-hash

hash1.rb

class Key
  attr_reader :value

  def initialize(value)
    @value = value
  end
end

k1 = Key.new('foo')
k2 = Key.new('foo')

h = { k1 => 'bar' }
puts h.has_key? k1 # -> true
puts h.has_key? k2 # -> false ??