function promiseAllStepN(n, list) {
  let tail = list.splice(n)
  let head = list
  let resolved = []
  let processed = 0
  return new Promise(resolve=>{
    head.forEach(x=>{
      let res = x()
      resolved.push(res)
      res.then(y=>{
        runNext()
        return y
      })
    })
    function runNext(){
      if(processed == tail.length){
        resolve(Promise.all(resolved))
      }else{
        resolved.push(tail[processed]().then(x=>{
          runNext()
          return x
        }))
        processed++
      }
    }
  })
}
Promise.allConcurrent = n => list =>  promiseAllStepN(n, list)