add(tRef, callback, isDeleteOnCall = true) {

    this.logger('debug', `A callback was registered for changes of "${tRef}"`);
    this.callbacks[tRef] = {
      execute: callback,
      isDeleteOnCall
    };

  }
The callback mechanism allows to dynamically listen to tiddler
changes without hardcoding a change-check for a tiddler name
in the refresh function.

@param {TiddlerReference} tRef - A tiddler whose change triggers
    the callback.
@param {function} callback - A function that is called when the
    tiddler has changed.
@param {boolean} [isDeleteOnCall=true] - True if to delete the
    callback once it has been called, false otherwise.