code: function (eventName, handler) {
      if (typeof(eventName) !== 'string') {
        throw new Error('Argument eventName should be a string');
      }
      if (typeof(handler) !== 'function') {
        throw new Error('Argument handler should be a function');
      }
      log('[Eventhandling] Adding event listener', eventName);
      this._validateEvent(eventName);
      this._handlers[eventName].push(handler);
    }
docs: Install an event handler for the given event name