text: discard """
    exitcode: 0
    output: '''
  1
  2
  3
  5
  '''
  """
  import asyncfutures
  
  let f1: Future[int] = newFuture[int]()
  f1.addCallback(proc() = echo 1)
  f1.addCallback(proc() = echo 2)
  f1.addCallback(proc() = echo 3)
  f1.complete(10)
  
  let f2: Future[int] = newFuture[int]()
  f2.addCallback(proc() = echo 4)
  f2.callback = proc() = echo 5
  f2.complete(10)
  