fix(core,store): child accounts share their parent's type; periods nest; lines checked when added

Three spec decisions from the owner (2026-10-13), each test-first:

- A child account has its parent's type (ParentTypeMismatch): a child
  subdivides its parent. Test: a_child_account_has_its_parents_type.
- Periods nest (a month in a quarter in a year) but never partially
  overlap (PeriodsOverlap; a refusal uses no id). Test:
  periods_nest_but_never_partially_overlap.
- A line's account must exist and take postings when the line is added,
  not only at post (UnknownAccount / PlaceholderAccount from add_line
  and add_line_fx); posting still re-checks. Two older tests that pinned
  the post-time refusal now check both (the post-time check is reached
  through the unchecked replay path, as old journals can).

Compatibility: replay reads history through new Ledger::replay_*
commands (add_account, add_line, add_line_fx, create_period) that skip
checks added later, so books written before these rules still open.
The contract test books_written_under_older_rules_still_open writes such
a journal directly and found, on its first run, that the SQLite
cross-check rebuild used the checked commands and would have failed
verification ("sqlite projection can't be read back"); it now replays
too. Documented in docs/spec/README.md; decisions recorded in rules.md
(B-1, B-4, B-12).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

crates/ledger-core/src/error.rs

@@ -123,6 +123,42 @@ pub enum LedgerError {
         spec: String,
     },
 
+    /// A child account subdivides its parent, so it must be the same type
+    /// (owner, 2026-10-13).
+    #[error(
+        "account {code} is {found:?}, but its parent {parent_code} is {expected:?}; a child account must be the same type as its parent"
+    )]
+    ParentTypeMismatch {
+        /// The new account's code.
+        code: String,
+        /// The parent's code.
+        parent_code: String,
+        /// The parent's type, which the child must share.
+        expected: crate::AccountType,
+        /// The type the child was given.
+        found: crate::AccountType,
+    },
+
+    /// Periods nest (a month in a quarter in a year) but never partially
+    /// overlap (owner, 2026-10-13).
+    #[error(
+        "period {name} ({start} to {end}) partially overlaps period {other} ({other_start} to {other_end}); periods may nest, but not overlap"
+    )]
+    PeriodsOverlap {
+        /// The new period's name.
+        name: String,
+        /// Its first day.
+        start: chrono::NaiveDate,
+        /// Its last day.
+        end: chrono::NaiveDate,
+        /// The existing period's name.
+        other: String,
+        /// The existing period's first day.
+        other_start: chrono::NaiveDate,
+        /// The existing period's last day.
+        other_end: chrono::NaiveDate,
+    },
+
     /// The referenced journal entry does not exist.
     #[error("entry {0} does not exist")]
     UnknownEntry(u64),

crates/ledger-core/src/ledger.rs

@@ -492,6 +492,22 @@ impl Ledger {
     }
 
     /// Add an account to the chart. Returns its id.
+    ///
+    /// ```text
+    /// GOAL: register an account under a unique code.
+    /// SOURCE: rules.md B-4 (the five elements); R-4; DECISION (owner,
+    ///         2026-10-13): a child has its parent's type.
+    /// SCENARIOS:
+    ///   1. IF the code is empty or taken: SHOULD refuse.
+    ///   2. IF the parent doesn't exist: SHOULD refuse.
+    ///   3. IF the parent is another type:
+    ///      - SHOULD refuse with ParentTypeMismatch
+    ///      - NEVER add the account
+    ///      BECAUSE a child subdivides its parent.
+    /// ```
+    ///
+    /// Replay of journals written before the parent rule uses
+    /// [`Self::replay_add_account`].
     pub fn add_account(
         &mut self,
         code: &str,
@@ -499,6 +515,30 @@ impl Ledger {
         account_type: AccountType,
         parent: Option<AccountId>,
         placeholder: bool,
+    ) -> Result<AccountId, LedgerError> {
+        if let Some(p) = parent.and_then(|p| self.chart.get(p))
+            && p.account_type != account_type
+        {
+            return Err(LedgerError::ParentTypeMismatch {
+                code: code.to_string(),
+                parent_code: p.code.clone(),
+                expected: p.account_type,
+                found: account_type,
+            });
+        }
+        self.replay_add_account(code, name, account_type, parent, placeholder)
+    }
+
+    /// [`Self::add_account`] without the rules added on 2026-10-13 (a
+    /// child's type), for replaying journals written before them. Never
+    /// for new work.
+    pub fn replay_add_account(
+        &mut self,
+        code: &str,
+        name: &str,
+        account_type: AccountType,
+        parent: Option<AccountId>,
+        placeholder: bool,
     ) -> Result<AccountId, LedgerError> {
         let id = self.next_account_id;
         self.next_account_id += 1;
@@ -616,6 +656,32 @@ impl Ledger {
         account: AccountId,
         side: Side,
         amount: Money,
+    ) -> Result<(), LedgerError> {
+        self.check_line_account(account)?;
+        self.replay_add_line(entry, account, side, amount)
+    }
+
+    /// A line's account must exist and take postings, checked when the
+    /// line is added (owner, 2026-10-13); posting checks again.
+    fn check_line_account(&self, account: AccountId) -> Result<(), LedgerError> {
+        let a = self
+            .chart
+            .get(account)
+            .ok_or(LedgerError::UnknownAccount(account))?;
+        if a.placeholder {
+            return Err(LedgerError::PlaceholderAccount(a.code.clone()));
+        }
+        Ok(())
+    }
+
+    /// [`Self::add_line`] without the account check added on 2026-10-13,
+    /// for replaying journals written before it. Never for new work.
+    pub fn replay_add_line(
+        &mut self,
+        entry: EntryId,
+        account: AccountId,
+        side: Side,
+        amount: Money,
     ) -> Result<(), LedgerError> {
         if amount.currency != self.base_currency {
             return Err(LedgerError::CurrencyMismatch {
@@ -648,6 +714,28 @@ impl Ledger {
         foreign_currency: Currency,
         foreign_amount: Decimal,
         fx_rate: Decimal,
+    ) -> Result<(), LedgerError> {
+        self.check_line_account(account)?;
+        self.replay_add_line_fx(
+            entry,
+            account,
+            side,
+            foreign_currency,
+            foreign_amount,
+            fx_rate,
+        )
+    }
+
+    /// [`Self::add_line_fx`] without the account check added on
+    /// 2026-10-13, for replaying journals written before it.
+    pub fn replay_add_line_fx(
+        &mut self,
+        entry: EntryId,
+        account: AccountId,
+        side: Side,
+        foreign_currency: Currency,
+        foreign_amount: Decimal,
+        fx_rate: Decimal,
     ) -> Result<(), LedgerError> {
         if foreign_amount <= Decimal::ZERO {
             return Err(LedgerError::InvalidDocumentLineAmount(foreign_amount));
@@ -919,11 +1007,55 @@ impl Ledger {
 
     /// Create an accounting period. The date range must be valid
     /// (end >= start). Returns the period id.
+    ///
+    /// ```text
+    /// GOAL: name a span of days that can later be closed.
+    /// SOURCE: rules.md B-12 (IRS Pub. 538: tax years, calendar or fiscal);
+    ///         DECISION (owner, 2026-10-13): periods nest, never
+    ///         partially overlap.
+    /// SCENARIOS:
+    ///   1. IF end is before start: SHOULD refuse.
+    ///   2. IF it lies inside, or around, every period it touches: SHOULD
+    ///      accept (a month in a quarter in a year).
+    ///   3. IF it partially overlaps a period:
+    ///      - SHOULD refuse with PeriodsOverlap
+    ///      - NEVER create it, or use up an id
+    ///      BECAUSE a span split across two periods could close twice or
+    ///      never.
+    /// ```
     pub fn create_period(
         &mut self,
         name: &str,
         start: NaiveDate,
         end: NaiveDate,
+    ) -> Result<PeriodId, LedgerError> {
+        periods::validate_period_range(start, end)?;
+        let nested = |a: (NaiveDate, NaiveDate), b: (NaiveDate, NaiveDate)| {
+            (a.0 >= b.0 && a.1 <= b.1) || (b.0 >= a.0 && b.1 <= a.1)
+        };
+        for q in self.list_periods() {
+            let touches = start <= q.end && q.start <= end;
+            if touches && !nested((start, end), (q.start, q.end)) {
+                return Err(LedgerError::PeriodsOverlap {
+                    name: name.to_string(),
+                    start,
+                    end,
+                    other: q.name.clone(),
+                    other_start: q.start,
+                    other_end: q.end,
+                });
+            }
+        }
+        self.replay_create_period(name, start, end)
+    }
+
+    /// [`Self::create_period`] without the nesting rule added on
+    /// 2026-10-13, for replaying journals written before it.
+    pub fn replay_create_period(
+        &mut self,
+        name: &str,
+        start: NaiveDate,
+        end: NaiveDate,
     ) -> Result<PeriodId, LedgerError> {
         periods::validate_period_range(start, end)?;
         let id = self.next_period_id;

crates/ledger-core/tests/invariants.rs

@@ -242,12 +242,18 @@ fn fx_overflow_is_an_error() {
 
 #[test]
 fn unknown_account_is_rejected() {
+    // Refused when the line is added (since 2026-10-13); a line that got
+    // in anyway (replayed from an older journal) is still refused at post.
     let c = fixture_company();
     let mut ledger = c.ledger;
     let id = ledger.create_entry(date(21), "Ghost account");
     ledger.add_line(id, c.cash, Side::Debit, usd(100)).unwrap();
+    assert_eq!(
+        ledger.add_line(id, 999_999, Side::Credit, usd(100)),
+        Err(LedgerError::UnknownAccount(999_999))
+    );
     ledger
-        .add_line(id, 999_999, Side::Credit, usd(100))
+        .replay_add_line(id, 999_999, Side::Credit, usd(100))
         .unwrap();
     assert_eq!(
         ledger.post_entry(id).unwrap_err(),
@@ -263,7 +269,14 @@ fn placeholder_account_is_rejected() {
         .add_account("100", "Assets header", AccountType::Asset, None, true)
         .unwrap();
     let id = ledger.create_entry(date(21), "Post to header");
-    ledger.add_line(id, header, Side::Debit, usd(100)).unwrap();
+    assert_eq!(
+        ledger.add_line(id, header, Side::Debit, usd(100)),
+        Err(LedgerError::PlaceholderAccount("100".to_string()))
+    );
+    // Got in anyway (an older journal): still refused at post.
+    ledger
+        .replay_add_line(id, header, Side::Debit, usd(100))
+        .unwrap();
     ledger
         .add_line(id, c.equity, Side::Credit, usd(100))
         .unwrap();
@@ -906,3 +919,100 @@ fn a_close_refuses_a_3999_that_is_not_equity() {
     assert_untouched(&before, &l);
     assert!(l.closed_period_containing(date(5)).is_none());
 }
+
+// ---------------------------------------------------------------------------
+// Chart, periods, and lines: rules confirmed by owner, 2026-10-13.
+// ---------------------------------------------------------------------------
+
+/// A child account subdivides its parent, so it is the same kind of
+/// account. SOURCE: rules.md B-4; DECISION (owner, 2026-10-13).
+#[test]
+fn a_child_account_has_its_parents_type() {
+    let mut l = Ledger::new(Currency::USD);
+    let cash = l
+        .add_account("1000", "Cash", AccountType::Asset, None, true)
+        .unwrap();
+    l.add_account("1010", "Checking", AccountType::Asset, Some(cash), false)
+        .unwrap();
+    let before = l.chart().len();
+    assert_eq!(
+        l.add_account("1020", "Rent", AccountType::Expense, Some(cash), false),
+        Err(LedgerError::ParentTypeMismatch {
+            code: "1020".into(),
+            parent_code: "1000".into(),
+            expected: AccountType::Asset,
+            found: AccountType::Expense,
+        })
+    );
+    assert_eq!(l.chart().len(), before);
+    assert!(l.chart().find_by_code("1020").is_none());
+}
+
+/// Periods nest (a month in a quarter in a year) but never partially
+/// overlap. DECISION (owner, 2026-10-13).
+#[test]
+fn periods_nest_but_never_partially_overlap() {
+    let d = |m: u32, day: u32| NaiveDate::from_ymd_opt(2026, m, day).unwrap();
+    let mut l = Ledger::new(Currency::USD);
+    l.create_period("2026-01", d(1, 1), d(1, 31)).unwrap();
+    l.create_period("2026-02", d(2, 1), d(2, 28)).unwrap();
+    // A quarter around both months, and a year around everything.
+    l.create_period("Q1 2026", d(1, 1), d(3, 31)).unwrap();
+    l.create_period("2026", d(1, 1), d(12, 31)).unwrap();
+    // A week inside January nests too.
+    l.create_period("week", d(1, 5), d(1, 11)).unwrap();
+    let next = l.next_period_id();
+    // Mid-January to mid-February straddles two months: refused.
+    assert!(matches!(
+        l.create_period("straddle", d(1, 15), d(2, 15)),
+        Err(LedgerError::PeriodsOverlap { .. })
+    ));
+    // So is a quarter that starts inside Q1.
+    assert!(matches!(
+        l.create_period("odd quarter", d(2, 1), d(4, 30)),
+        Err(LedgerError::PeriodsOverlap { .. })
+    ));
+    assert_eq!(l.next_period_id(), next, "refused means no id burned");
+    // Disjoint is fine.
+    l.create_period(
+        "2027-01",
+        NaiveDate::from_ymd_opt(2027, 1, 1).unwrap(),
+        NaiveDate::from_ymd_opt(2027, 1, 31).unwrap(),
+    )
+    .unwrap();
+}
+
+/// A line names a real, postable account when it is added, not only when
+/// the entry posts. DECISION (owner, 2026-10-13); rules.md B-1.
+#[test]
+fn a_line_on_an_unknown_or_placeholder_account_is_refused_when_added() {
+    let c = fixture_company();
+    let mut l = c.ledger.clone();
+    let header = l
+        .add_account("1900", "Other assets", AccountType::Asset, None, true)
+        .unwrap();
+    let e = l.create_entry(date(21), "Lines");
+    assert_eq!(
+        l.add_line(e, 999_999, Side::Debit, usd(10)),
+        Err(LedgerError::UnknownAccount(999_999))
+    );
+    assert_eq!(
+        l.add_line(e, header, Side::Debit, usd(10)),
+        Err(LedgerError::PlaceholderAccount("1900".into()))
+    );
+    assert!(matches!(
+        l.add_line_fx(
+            e,
+            header,
+            Side::Debit,
+            Currency::EUR,
+            Decimal::from(10),
+            Decimal::from(2)
+        ),
+        Err(LedgerError::PlaceholderAccount(_))
+    ));
+    assert!(
+        l.get_entry(e).unwrap().lines().is_empty(),
+        "refused lines aren't added"
+    );
+}

crates/ledger-store/src/store.rs

@@ -2374,6 +2374,10 @@ impl Store {
     }
 
     /// Rebuild a ledger purely from the SQLite projection (used by verify).
+    /// Rebuild the ledger from the SQLite projection, for the cross-check.
+    /// Like journal replay, it reads history, so it uses the engine's
+    /// `replay_*` commands: rules added later never refuse books written
+    /// before them.
     fn load_ledger_from_sqlite(&self) -> Result<Ledger, StoreError> {
         let mut ledger = Ledger::new(self.base_currency);
         let mut stmt = self.conn.prepare(
@@ -2400,7 +2404,7 @@ impl Store {
                     "account {code} currency {currency} != base"
                 )));
             }
-            let got = ledger.add_account(
+            let got = ledger.replay_add_account(
                 &code,
                 &name,
                 account_type,
@@ -2477,7 +2481,7 @@ impl Store {
                     })?;
                     for (idx, lr) in lrows.enumerate() {
                         let (account, side, amount, currency, class_id) = lr?;
-                        ledger.add_line(
+                        ledger.replay_add_line(
                             got,
                             account as u64,
                             parse_side(&side)?,
@@ -2883,7 +2887,7 @@ pub(crate) fn replay_line_into_ledger(
             let want = get_u64(p, "id")?;
             let account_type: AccountType = serde_json::from_value(p["account_type"].clone())?;
             let parent = p.get("parent").and_then(|v| v.as_u64());
-            let got = ledger.add_account(
+            let got = ledger.replay_add_account(
                 get_str(p, "code")?,
                 get_str(p, "name")?,
                 account_type,
@@ -2928,7 +2932,7 @@ pub(crate) fn replay_line_into_ledger(
                     let fx_rate: Decimal = get_str(p, "fx_rate")?
                         .parse()
                         .map_err(|_| StoreError::JournalCorrupt("bad fx rate".into()))?;
-                    ledger.add_line_fx(
+                    ledger.replay_add_line_fx(
                         id,
                         account,
                         side,
@@ -2940,7 +2944,7 @@ pub(crate) fn replay_line_into_ledger(
                     )?;
                 }
                 None => {
-                    ledger.add_line(id, account, side, Money::new(amount, currency))?;
+                    ledger.replay_add_line(id, account, side, Money::new(amount, currency))?;
                 }
             }
         }
@@ -2957,7 +2961,7 @@ pub(crate) fn replay_line_into_ledger(
                 for l in p.get("lines").and_then(|v| v.as_array()).ok_or_else(|| {
                     StoreError::JournalCorrupt("entry.posted without lines".into())
                 })? {
-                    ledger.add_line(
+                    ledger.replay_add_line(
                         id,
                         get_u64(l, "account")?,
                         parse_side(get_str(l, "side")?)?,
@@ -3132,7 +3136,7 @@ pub(crate) fn replay_line_into_ledger(
         }
         "period.created" => {
             let want = get_u64(p, "id")?;
-            let got = ledger.create_period(
+            let got = ledger.replay_create_period(
                 get_str(p, "name")?,
                 parse_date(get_str(p, "start")?)?,
                 parse_date(get_str(p, "end")?)?,

crates/ledger-store/tests/store.rs

@@ -1000,3 +1000,55 @@ mod refusals {
         }
     }
 }
+
+/// Contract (2026-10-13): rules added to the engine never lock anyone out
+/// of books written before them. A journal holding what the new rules
+/// refuse (a child account of another type, partially overlapping periods,
+/// a draft line on a missing account) must still open and verify; replay
+/// goes through Ledger::replay_*.
+#[test]
+fn books_written_under_older_rules_still_open() {
+    use serde_json::json;
+    let dir = test_dir("older-rules");
+    let mut store = Store::create(&dir, Currency::USD).unwrap();
+    let cash = store
+        .add_account("1000", "Cash", AccountType::Asset, None, true)
+        .unwrap();
+    drop(store);
+    // What an older wanderbooks could have written.
+    let mut w = JournalWriter::open(&dir.join(ledger_store::JOURNAL_FILE)).unwrap();
+    w.append(
+        "account.added",
+        json!({"id": 2, "code": "1010", "name": "Rent", "account_type": "Expense",
+               "parent": cash, "placeholder": false, "currency": "USD"}),
+    )
+    .unwrap();
+    w.append(
+        "period.created",
+        json!({"id": 1, "name": "2026-01", "start": "2026-01-01", "end": "2026-01-31"}),
+    )
+    .unwrap();
+    w.append(
+        "period.created",
+        json!({"id": 2, "name": "straddle", "start": "2026-01-15", "end": "2026-02-15"}),
+    )
+    .unwrap();
+    w.append(
+        "entry.created",
+        json!({"id": 1, "date": "2026-03-01", "description": "draft"}),
+    )
+    .unwrap();
+    w.append(
+        "entry.line_added",
+        json!({"id": 1, "account": 999, "side": "debit", "amount": "10.00", "currency": "USD"}),
+    )
+    .unwrap();
+    drop(w);
+    // Opening rebuilds from the journal; it must accept all of it.
+    let _ = std::fs::remove_file(dir.join(ledger_store::DB_FILE));
+    let store = Store::open(&dir).expect("older books still open");
+    store.verify().unwrap();
+    assert_eq!(store.ledger().chart().len(), 2);
+    assert_eq!(store.ledger().list_periods().len(), 2);
+    assert_eq!(store.ledger().get_entry(1).unwrap().lines().len(), 1);
+}

docs/spec/README.md

@@ -31,6 +31,14 @@ SOURCE: CONVENTION (QuickBooks aging); nearest authority ASC 326-20-55-38 (illus
 A rule whose only source is CONVENTION or DECISION is a candidate for a
 human to confirm or overturn; nothing else is.
 
+## Rules added later never lock out older books
+
+When a rule is tightened (a check added to a command), books written
+before it must still open. Journal replay and the SQLite cross-check read
+history through the engine's `replay_*` commands, which skip checks added
+later; new commands go through the checked ones. Pinned by
+`books_written_under_older_rules_still_open` (ledger-store).
+
 ## Vendoring rules
 
 - Public-domain texts are copied **verbatim** (whitespace normalized), each

docs/spec/rules.md

@@ -27,7 +27,10 @@ Spec blocks in the code cite these as `SOURCE: rules.md B-n`.
   total credits after you post the journal entries to the ledger accounts."
 
 **Enforced:** `kernel::check_lines` via `JournalEntry::validate` (two or
-more lines, debits = credits exactly). **Checked:** Kani
+more lines, debits = credits exactly). A line's account must exist and
+take postings when the line is added, not only at post: DECISION
+(the owner, 2026-10-13), `Ledger::add_line`
+(`a_line_on_an_unknown_or_placeholder_account_is_refused_when_added`). **Checked:** Kani
 `check_lines_is_exact`, `accepted_entry_nets_to_zero`; Lean
 `Accepted.net_zero`; property `prop_posted_entries_balance`.
 
@@ -74,7 +77,9 @@ its liabilities … from delivering or producing goods, rendering services,
 or carrying out other activities." E81 expenses "are outflows or other
 using up of assets … or incurrences of its liabilities …".
 
-**Enforced:** `AccountType` (five variants). **Checked:** by type.
+**Enforced:** `AccountType` (five variants). A child account has its
+parent's type: DECISION (the owner, 2026-10-13), `Ledger::add_account`
+(`ParentTypeMismatch`). **Checked:** `a_child_account_has_its_parents_type`.
 
 ## B-5 Normal balances
 
@@ -178,7 +183,10 @@ the entity
 Periods themselves: IRS Pub. 538 (`sources/irs-periods-and-methods.md`)
 defines the tax year, calendar and fiscal. Locking a closed period and
 closing in order are practice (and follow from B-8: a closed period's
-book is not rewritten). **Enforced:** `closed_period_containing`,
+book is not rewritten). Periods nest (a month in a quarter in a year)
+and never partially overlap: DECISION (the owner, 2026-10-13),
+`Ledger::create_period` (`PeriodsOverlap`;
+`periods_nest_but_never_partially_overlap`). **Enforced:** `closed_period_containing`,
 `CloseOutOfOrder`, `Store::rehearse`. **Checked:** Lean `Steps.frozen`;
 store property `refused_commands_never_reach_the_journal` (no draft in a
 closed period).