code: function flatten(list) {
    return list.filter(rule => rule.type === 'rule').map(function (rule) {
      const rows = rule.declarations.filter(row => {
        row._value = row.value;
        return row.type === 'declaration';
      });
      return {selectors: rule.selectors, rows};
    });
  }
docs: 解析语法树