fun vanEck ← void by int firstIndex, int lastIndex
  logic isFirst ← true
  Map map ← int%int[]
  int last ← 0
  write("[")
  if firstIndex æ 1
    write("0")
    isFirst ← false
  end
  for int n ← 2 ; n ≤ lastIndex ; ++n
    int v ← when(map.has(last), n - map[last], 0)
    map[last] ← n
    last ← v
    if n ≥ firstIndex
      write(when(isFirst, Text.EMPTY, ",") + v)
      isFirst ← false
    end
  end
  writeLine("]")
end
Pair[int%int(1 ⇒ 10), int%int(991 ⇒ 1000)].list(
void by var pair
  writeLine("Terms " + pair.key + " to " +
    pair.value + " of the sequence are:")
  vanEck(pair.key, pair.value)
  writeLine()
end)