/**
   * Returns the last element of an observable sequence that satisfies the condition in the predicate if specified, else the last element.
   * @param {Function} [predicate] A predicate function to evaluate for elements in the source sequence.
   * @param {Any} [thisArg] Object to use as `this` when executing the predicate.
   * @returns {Observable} Sequence containing the last element in the observable sequence that satisfies the condition in the predicate.
   */
  observableProto.last = function (predicate, thisArg) {
    return predicate ?
      this.where(predicate, thisArg).last() :
      lastOrDefaultAsync(this, false);
  };