/**
   * Returns the elements in an observable sequence with the maximum  key value according to the specified comparer.
   * @example
   * var res = source.maxBy(function (x) { return x.value; });
   * var res = source.maxBy(function (x) { return x.value; }, function (x, y) { return x - y;; });
   * @param {Function} keySelector Key selector function.
   * @param {Function} [comparer]  Comparer used to compare key values.
   * @returns {Observable} An observable sequence containing a list of zero or more elements that have a maximum key value.
   */
  observableProto.maxBy = function (keySelector, comparer) {
    comparer || (comparer = defaultSubComparer);
    return extremaBy(this, keySelector, comparer);
  };