# File lib/mongo/id.rb, line 51
    def self.included(klass)
      klass.class_variable_set(:@@id, 0)
      klass.class_variable_set(:@@id_lock, Mutex.new)

      klass.define_singleton_method(:next_id) do
        klass.class_variable_get(:@@id_lock).synchronize do
          id = class_variable_get(:@@id)
          klass.class_variable_set(:@@id, id + 1)
          klass.class_variable_get(:@@id)
        end
      end
    end