while
    set $time to current time
    test $time < $expiry
        fail: break loop
    test $time - $prevtime >= 10
        fail: sleep 10

  $prevtime = $time
  warn $time

##</code><code>##

$expiry = 3600
$prevtime = undef
$time = undef

1st loop:
    $time = 123
    $time 123 < $expiry 3600 (true)
    $time 123 - $prevtime undef >= 10 (true)
  $prevtime = 123
  warn $time 123

2nd loop:
    $time = 123 (first iteration takes less than 1 second)
    $time 123 < $expiry 3600 (true)
    $time 123 - $prevtime 123 >= 10 (false)
    sleep 10
  $prevtime = 123
  warn $time 123

3rd loop:
    $time = 133 (slept on last iteration so advance 10s)
    $time 133 < $expiry 3600 (true)
    $time 133 - $prevtime 123 >= 10 (true)
  $prevtime = 133
  warn $time 133

4th loop:
    $time = 133 (last iteration took less than 1s)
    ...