/**
   * Returns an observable sequence containing a value that represents how many elements in the specified observable sequence satisfy a condition if provided, else the count of items.
   * @example
   * res = source.count();
   * res = source.count(function (x) { return x > 3; });
   * @param {Function} [predicate]A function to test each element for a condition.
   * @param {Any} [thisArg] Object to use as this when executing callback.
   * @returns {Observable} An observable sequence containing a single element with a number that represents how many elements in the input sequence satisfy the condition in the predicate function if provided, else the count of items in the sequence.
   */
  observableProto.count = function (predicate, thisArg) {
    return predicate ?
      this.filter(predicate, thisArg).count() :
      this.reduce(function (count) { return count + 1; }, 0);
  };