old_contents: module.exports = {
    conditions: [''],
    name: 'IdentifierStart',
    rules: [
      `UnicodeIDStart`,
      `UnicodeEscapeSequence`,
    ],
    handlers: [
      `$$ = $1;`,
      `$$ = $1;`, // TODO: unicode with id start
    ],
    subRules: [
      require('./UnicodeEscapeSequence'),
    ],
  };
  
new_contents: module.exports = {
    conditions: [''],
    name: 'IdentifierStart',
    rules: [
      `UnicodeIDStart`,
      `UnicodeEscapeSequence`,
      `$`,
      `_`,
    ],
    handlers: [
      `$$ = $1;`,
      `$$ = $1;`, // TODO: unicode with id start
      `$$ = $1;`,
      `$$ = $1;`,
    ],
    subRules: [
      require('./UnicodeEscapeSequence'),
    ],
  };
  
subject: add dollar and underscore