Grain conveyor pull-cord — a latched trip, and a broken safety circuit that stays broken

conveyor_pullcord.cell

// GRAIN CONVEYOR PULL-CORD — a latched trip, and a broken safety
// circuit that stays broken. Grain handling kills dozens every year;
// engulfment victims are often pulled in by a conveyor a coworker
// restarted while they were still in the boot pit. Two properties are
// structural here. Anti-restart: a pull-cord trip latches, and `tripped`
// has NO edge to `running` — the only exit is `reset_walked`, a state a
// human reaches by walking the belt line. Fail-safe wiring: the pull-
// cord loop is supervised, and a severed wire lands in `circuit_fault`,
// a TERMINAL state soma verify proves has no exit — a conveyor whose
// safety circuit is cut can never be software-restarted.

cell ConveyorGuard {
    face {
        signal start_belt(id: String) -> Map
        signal stop_belt(id: String) -> String
        signal pull_cord(id: String) -> String
        signal walk_the_line(id: String) -> String
        signal wire_severed(id: String) -> String
    }

    memory {
        // completed walkdowns since the last trip — the reset evidence
        walkdowns: Map<String, Int> [persistent, consistent]
        invariant walkdowns >= 0 && walkdowns <= 1000
    }

    state belt {
        initial: stopped
        stopped -> running
        running -> stopped          // normal stop
        running -> tripped          // pull-cord: LATCHED
        tripped -> reset_walked     // a person walked the full belt line
        reset_walked -> stopped     // re-arm; start is a separate command
        running -> circuit_fault    // supervision saw the loop go open
        stopped -> circuit_fault
        tripped -> circuit_fault
    }

    on start_belt(id: String) {
        let r = try { transition(id, "running") }
        if r.error != () {
            return map("ok", false, "state", get_status(id),
                       "reason", "belt is latched out — no restart from here")
        }
        return map("ok", true, "state", get_status(id))
    }

    on stop_belt(id: String) { transition(id, "stopped")  return get_status(id) }
    on pull_cord(id: String) { transition(id, "tripped")  return get_status(id) }

    on walk_the_line(id: String) {
        transition(id, "reset_walked")
        walkdowns.set(id, (walkdowns.get(id) ?? 0) + 1)
        transition(id, "stopped")
        return get_status(id)
    }

    on wire_severed(id: String) { transition(id, "circuit_fault")  return get_status(id) }
}

cell test ConveyorTests {
    rules {
        // normal run and stop
        assert start_belt("c1").ok == true
        assert stop_belt("c1") == "stopped"

        // pull-cord trip LATCHES: no restart, the edge is absent
        assert start_belt("c1").ok == true
        assert pull_cord("c1") == "tripped"
        assert start_belt("c1").ok == false
        assert_fails transition("c1", "running")

        // only a walked line releases the latch, then a fresh start
        assert walk_the_line("c1") == "stopped"
        assert start_belt("c1").ok == true
        assert stop_belt("c1") == "stopped"

        // a severed pull-cord wire is TERMINAL — proven, and pinned here
        assert wire_severed("c1") == "circuit_fault"
        assert start_belt("c1").ok == false
        assert_fails transition("c1", "stopped")
        assert_fails transition("c1", "reset_walked")

        // the walkdown ledger never goes negative
        assert_fails walkdowns.set("c1", -1)
    }
}