code: function (event, preferenceID, handler) {
              if (typeof preferenceID === "function") {
                  handler = preferenceID;
                  preferenceID = null;
              }
  
              if (preferenceID) {
                  var pref = this.getPreference(preferenceID);
                  pref.on(event, handler);
              } else {
                  this._installListener();
                  this._on_internal(event, handler);
              }
          }
docs: Sets up a listener for events for this PrefixedPreferencesSystem. Only prefixed events
  will notify. Optionally, you can set up a listener for a specific preference.
  
  @param {string} event Name of the event to listen for
  @param {string|Function} preferenceID Name of a specific preference or the handler function
  @param {?Function} handler Handler for the event