code: function checkIdentifier(str, context) {
      if (!/^[a-zA-Z_][a-zA-Z_0-9]*$/.test(str)) {
          throw new ImplementationError(`Invalid identifier "${str}"${context.errorContext}`);
      }
      return str;
  }
docs: Generates an error for invalid identifier strings. Identifiers contain letters,
  numbers and underscore - and do not start with a number.
  
  @private