code: function bundle () {
    const ordered = order(deps[postcssify.entry], [], {})
    const l = ordered.length
    let ast
    for (let i = 0; i < l; i++) {
      const file = ordered[i]
      const style = styles[file]
      if (style === void 0) {
        return
      } else {
        if (!ast) {
          ast = style.clone()
        } else {
          ast.append(style)
        }
      }
    }
    if (ast) {
      postcss(processors).process(ast.toResult(), { to: dest, map: true }).then((result) => {
        fs.writeFile(dest, result.css, (err) => {
          if (err) {
            throw err
          } else {
            log.info('updated: %s', dest)
          }
        })
      }).catch((err) => log.error(err))
    }
  }
docs: concat and bundle css