docs: async function stop () {
      return new Promise(function (resolve, reject) {
          // if not server then resolve
          if (!immutableApp || !immutableApp.server || !immutableApp.started) {
              resolve()
          }
          // close server
          immutableApp.server.close(function (err) {
              if (err) {
                  reject(err)
              }
              else {
                  // clear server
                  immutableApp.server = undefined
                  immutableApp.started = false
                  // resolve
                  resolve()
              }
          })
      })
  }