/**
   * Returns the maximum value in an observable sequence according to the specified comparer.
   * @example
   * var res = source.max();
   * var res = source.max(function (x, y) { return x.value - y.value; });
   * @param {Function} [comparer] Comparer used to compare elements.
   * @returns {Observable} An observable sequence containing a single element with the maximum element in the source sequence.
   */
  observableProto.max = function (comparer) {
    return this.maxBy(identity, comparer).map(function (x) { return firstOnly(x); });
  };