on hailstone (n, sequenceList)
  len = 1
  repeat while n<>1
    if listP(sequenceList) then sequenceList.add(n)
    if n mod 2 = 0 then
      n = n / 2
    else
      n = 3 * n + 1
    end if
    len = len + 1
  end repeat
  if listP(sequenceList) then sequenceList.add(n)
  return len
end