Trying to understand Nim awaitAny()

awaitAny_tryout.nim

import threadpool, os, strformat

proc timer(d: int): int =
  echo fmt"sleeping {d}"
  sleep(d)
  echo fmt"done {d}"
  return d

var durations = [1000, 2000, 3000, 4000, 5000]
var tasks: seq[FlowVarBase] = @[]
var results: seq[int] = @[]

for i in 0 .. durations.high:
  tasks.add spawn timer(durations[i])

var index = awaitAny(tasks)
while index != -1:
  results.add ^cast[FlowVar[int]](tasks[index])
  tasks.del(index)
  echo repr results
  index = awaitAny(tasks) 

sync()