Two readings, two reactions — and one rule that knows when to hold back.
Onno's watch on the bell tower is, in his telling, mostly sitting. The sensor on the outer wall does the work. It reports the temperature, the wind speed, and whether the storm-gate is barred — all through a small chime that hangs by his elbow. He's written down two rules so that even on the nights he dozes, the right things happen.
The first rule, for the brazier: ? the temperature drops below freezing, light it. Reactive — it just waits.
The second rule, for the shutters: close them when the wind picks up past thirty — ? the storm-gate is already barred, in which case the wall takes the wind and the shutters can stay open.
About these verbs
when waits for a condition to become true, then runs its action. unless is a guard you attach to a when — it holds the action back while the guard condition is true. So when X unless Y reads as "do this when X happens, but not while Y." unless never stands on its own; it always follows a when.
_____ temperature is below 0:
show "light the brazier"
when wind is above 30 _____ gate is barred:
show "close the shutters"
reading → temp -3 · wind 35 · gate open
when (cold) fires → light the brazier
when (windy), gate open → close the shutters
if gate were barred → shutters stay open — the guard holds