// entry action; can start at any point in list
test(initial_stamp)

// recursive transit of list
// will either error, or exit due to EndOfList
function test(current)
  next := nextmarker(current)
  if next - current > FifteenMinutes then
    error
  else
    test(next)
  endif
endfunc

// get next 15m marker
// returns marker or exits due to EndOfList
function nextmarker(stamp)
  check_stamp := stamp.next
  if check_stamp == EndOfList
    exit
  if check_stamp MOD FifteenMinutes <> 0 then
    return nextmarker(check_stamp)
  else
    return check_stamp
  endif
endfunc

function test(current)
  next := nextmarker(current)
  while next - current <= FifteenMinutes do
    current := next
    next := nextmarker(current)
  done
  error
endfunc