code: function createSyntaxError (message, c) {
    if (c === undefined) {
      c = index - token.length;
    }
    let error = new SyntaxError(message + ' (char ' + c + ')');
    error['char'] = c;
  
    return error;
  }
docs: Create an error
  @param {string} message
  @param {number} [c]  Optional index (character position) where the
  error happened. If not provided, the start of
  the current token is taken
  @return {SyntaxError} instantiated error
  @private