def subscribe_param(key)
      code, message, uri = @proxy.subscribeParam(@caller_id, @slave_uri, key)
      if code == 1
        return true
      else
        raise message
      end
    end
def subscribe_param(key)
      code, message, uri = @proxy.subscribeParam(@caller_id, @slave_uri, key)
      if code == 1
        return true
      else
        raise message
      end
    end
this method is not described in the wiki.
 subscribe to the parameter key.
 @param [String] key name of parameter
 @return [Boolean] true
 @raise [RuntimeError] if fail