old_contents: 
  // TODO: instanceof return false.
  
  /**
   *  Error class specific to `Notation`.
   *  @name Notation.Error
   *  @memberof! Notation
   *  @class
   *
   */
  class NotationError extends Error {
  
      /**
       *  Initializes a new `Notation.Error` instance.
       *  @constructs Notation.Error
       *  @param {String} message - The error message.
       */
      constructor(message = '') {
          super(message);
          this.name = this.constructor.name;
  
          Object.defineProperty(this, 'name', {
              enumerable: false,
              writable: false,
              value: 'NotationError'
          });
  
          Object.defineProperty(this, 'message', {
              enumerable: false,
              writable: true,
              value: message
          });
  
          // V8
          if (Error.hasOwnProperty('captureStackTrace')) { // eslint-disable-line no-prototype-builtins
              Error.captureStackTrace(this, this.constructor);
          } else {
              Object.defineProperty(this, 'stack', {
                  enumerable: false,
                  writable: false,
                  value: (new Error(message)).stack
              });
          }
      }
  
  }
  
  export default NotationError;
  
new_contents: /* eslint consistent-this:0, no-prototype-builtins:0 */
  
  const setProto = Object.setPrototypeOf;
  
  /**
   *  Error class specific to `Notation`.
   *  @name Notation.Error
   *  @memberof! Notation
   *  @class
   *
   */
  class NotationError extends Error {
  
      /**
       *  Initializes a new `Notation.Error` instance.
       *  @constructs Notation.Error
       *  @param {String} message - The error message.
       */
      constructor(message = '') {
          super(message);
          setProto(this, NotationError.prototype);
  
          Object.defineProperty(this, 'name', {
              enumerable: false,
              writable: false,
              value: 'NotationError'
          });
  
          Object.defineProperty(this, 'message', {
              enumerable: false,
              writable: true,
              value: message
          });
  
          /* istanbul ignore else */
          if (Error.hasOwnProperty('captureStackTrace')) { // V8
              Error.captureStackTrace(this, NotationError);
          } else {
              Object.defineProperty(this, 'stack', {
                  enumerable: false,
                  writable: false,
                  value: (new Error(message)).stack
              });
          }
      }
  
  }
  
  export default NotationError;
  
subject: Fix instanceof NotationError