docs: function checkCallbacks(start, arrayOfCallbacks) {
              // TODO: Promises/A+ update type checking and docs
              var arg, i = arrayOfCallbacks.length;
  
              while (i > start) {
                  arg = arrayOfCallbacks[--i];
  
                  if (arg != null && typeof arg != 'function') {
                      throw new Error('arg ' + i + ' must be a function');
                  }
              }
          }