# File lib/preferences.rb, line 209
  def register_pref_key(obj, key)
    obj_id = obj.object_id
    if metadata[obj_id]
      unless Metadata.normalize(key) == metadata[obj_id].key
        raise ConfigError,
          "Cannot register #{key}. Already registered a preferences key " +
          "for this object, #{metadata[obj_id].key}."
      end
    else
      metadata[obj_id] = Metadata.new(preferences, key)
    end
    key
  end