code: function appendPrefixedTopic (arr, topic) {
      var prefix = arr[arr.length - 1] === ''
        ? '' : arr[arr.length - 1] + '.';
  
      arr.push(prefix + topic);
      return arr;
    }
docs: Appends the fully qualified topic name to the array using the last element of the array. This array must have length 1. If it should start at the root, then the array should be [ '' ].