  /**
   * Returns the only element of an observable sequence that matches the predicate, or a default value if no such element exists; this method reports an exception if there is more than one element in the observable sequence.
   * @example
   * var res = res = source.singleOrDefault();
   * var res = res = source.singleOrDefault(function (x) { return x === 42; });
   * res = source.singleOrDefault(function (x) { return x === 42; }, 0);
   * res = source.singleOrDefault(null, 0);
   * @memberOf Observable#
   * @param {Function} predicate A predicate function to evaluate for elements in the source sequence.
   * @param [defaultValue] The default value if the index is outside the bounds of the source sequence.
   * @param {Any} [thisArg] Object to use as `this` when executing the predicate.        
   * @returns {Observable} Sequence containing the single element in the observable sequence that satisfies the condition in the predicate, or a default value if no such element exists.
   */
  observableProto.singleOrDefault = function (predicate, defaultValue, thisArg) {
    return predicate && isFunction(predicate) ?
      this.where(predicate, thisArg).singleOrDefault(null, defaultValue) :
      singleOrDefaultAsync(this, true, defaultValue);
  };