when (NimMajor, NimMinor) < (1, 4):
  {.push raises: [Defect].}
else:
  {.push raises: [].}

proc flatten*[T](a: seq[seq[T]]): seq[T] =
  var aFlat = newSeq[T](0)
  for subseq in a:
    aFlat &= subseq
  return aFlat