Bonded warehouse — no duty, no exit

bonded_warehouse.cell

// ============================================================================
// BONDED WAREHOUSE — no duty, no exit
// ============================================================================
//
// A customs bonded warehouse holds imported goods on which duty has NOT
// yet been paid. The whole institution rests on one rule: bonded goods
// leave for the domestic market only after duty is settled, or leave the
// country entirely (re-export, duty never owed). "Bond leakage" — goods
// slipping into commerce duty-unpaid — is a perennial fraud; the UK lost
// hundreds of millions of pounds a year to bonded alcohol diversion in
// the 1990s "London City Bond" scandal, where paperwork showed exports
// that never left the island.
//
// Structurally: `domestic_market` is reachable ONLY through `duty_paid`,
// `re_exported` is terminal, and the duty ledger refuses a release whose
// payment doesn't cover the assessment. The diversion has no path.
// ============================================================================

cell BondedWarehouse {
    face {
        signal admit_goods(consignment: String, duty_owed_cents: Int) -> Map
        signal pay_duty(consignment: String, cents: Int) -> Map
        signal release_domestic(consignment: String) -> Map
        signal re_export(consignment: String) -> Map
        signal bond_status(consignment: String) -> Map
    }

    memory {
        owed: Map<String, Int> [persistent, consistent]
        invariant owed >= 0

        paid: Map<String, Int> [persistent, consistent]
        invariant paid >= 0
    }

    // No edge in_bond -> domestic_market. Duty first, always.
    // Both exits are terminal: goods leave the bond exactly once.
    state bond {
        initial: in_bond
        in_bond   -> duty_paid
        in_bond   -> re_exported
        duty_paid -> domestic_market
    }

    on admit_goods(consignment: String, duty_owed_cents: Int) {
        if duty_owed_cents <= 0 { return map("ok", false, "reason", "assessment must be positive") }
        owed.set(consignment, duty_owed_cents)
        paid.set(consignment, 0)
        return map("ok", true, "consignment", consignment, "owed_cents", duty_owed_cents)
    }

    on pay_duty(consignment: String, cents: Int) {
        if cents <= 0 { return map("ok", false, "reason", "payment must be positive") }
        let assessment = owed.get(consignment)
        if assessment == () { return map("ok", false, "reason", "unknown consignment") }
        let total = (paid.get(consignment) ?? 0) + cents
        paid.set(consignment, total)
        if total < assessment {
            let short = assessment - total
            return map("ok", true, "settled", false, "short_cents", short)
        }
        // settled in full — NOW the gate opens
        let r = try { transition(consignment, "duty_paid") }
        if r.error != () { return map("ok", false, "reason", "{r.error}") }
        return map("ok", true, "settled", true, "state", "duty_paid")
    }

    on release_domestic(consignment: String) {
        let r = try { transition(consignment, "domestic_market") }
        if r.error != () {
            return map("ok", false, "reason", "duty not settled: {r.error}")
        }
        return map("ok", true, "state", "domestic_market")
    }

    on re_export(consignment: String) {
        let r = try { transition(consignment, "re_exported") }
        if r.error != () { return map("ok", false, "reason", "{r.error}") }
        return map("ok", true, "state", "re_exported")
    }

    on bond_status(consignment: String) {
        return map("consignment", consignment, "state", get_status(consignment),
                   "owed_cents", owed.get(consignment) ?? 0,
                   "paid_cents", paid.get(consignment) ?? 0)
    }
}

cell test NoBondLeakage {
    rules {
        // 40 cases of whisky enter the bond owing 120,000 cents of duty
        assert admit_goods("WX-40", 120000).ok == true

        // THE DIVERSION: straight to the domestic market, duty unpaid.
        // The edge does not exist.
        assert release_domestic("WX-40").ok == false
        assert_fails transition("WX-40", "domestic_market")

        // partial payment does not open the gate
        assert pay_duty("WX-40", 50000).settled == false
        assert release_domestic("WX-40").ok == false

        // full settlement does
        assert pay_duty("WX-40", 70000).settled == true
        assert release_domestic("WX-40").ok == true
        assert bond_status("WX-40").state == "domestic_market"

        // released goods cannot re-enter the bond or "re-export"
        assert_fails transition("WX-40", "in_bond")
        assert_fails transition("WX-40", "re_exported")

        // re-export is legal duty-free — and terminal (the LCB fraud was
        // "exported" goods reappearing; here they cannot re-enter)
        assert admit_goods("WX-41", 99000).ok == true
        assert re_export("WX-41").ok == true
        assert_fails transition("WX-41", "in_bond")
        assert_fails transition("WX-41", "duty_paid")
        assert release_domestic("WX-41").ok == false

        // ledgers refuse negative money
        assert_fails owed.set("WX-41", -1)
        assert_fails paid.set("WX-41", -1)
    }
}