function assertExtensions(should) {
  var i = should.format;

  /*
   * Expose assert to should
   *
   * This allows you to do things like below
   * without require()ing the assert module.
   *
   *    should.equal(foo.bar, undefined);
   *
   */
  merge(should, assert);

  /**
   * Assert _obj_ exists, with optional message.
   *
   * @static
   * @memberOf should
   * @category assertion assert
   * @alias should.exists
   * @param {*} obj
   * @param {String} [msg]
   * @example
   *
   * should.exist(1);
   * should.exist(new Date());
   */
  should.exist = should.exists = function(obj, msg) {
    if (null == obj) {
      throw new AssertionError({
        message: msg || "expected " + i(obj) + " to exist",
        stackStartFunction: should.exist
      });
    }
  };

  should.not = {};
  /**
   * Asserts _obj_ does not exist, with optional message.
   *
   * @name not.exist
   * @static
   * @memberOf should
   * @category assertion assert
   * @alias should.not.exists
   * @param {*} obj
   * @param {String} [msg]
   * @example
   *
   * should.not.exist(null);
   * should.not.exist(void 0);
   */
  should.not.exist = should.not.exists = function(obj, msg) {
    if (null != obj) {
      throw new AssertionError({
        message: msg || "expected " + i(obj) + " to not exist",
        stackStartFunction: should.not.exist
      });
    }
  };
}
Node.js standard [`assert.ifError`](http://nodejs.org/api/assert.html#assert_assert_iferror_value).
@static
@memberOf should
@category assertion assert
@param {Error} err