closes: a run of members closes splits cumulatively

Each close into members' capital rounded on its own, ties to the
earlier member, so twelve monthly 50/50 closes of odd-cent income could
leave one member several cents ahead of half the year (docs/testing.md
#12, found with the members' equity fix).

- CloseTo::Members now splits cumulatively over a run (closes back to
  back with the same members and shares): each close gives a member
  allocate(run total so far) - allocate(run total before). Periods
  record their close order and a cumulative close's members and net
  (Period::close_seq, member_close) so the next close continues the run
  and replay recomputes it exactly. A close to anything else, or other
  shares, ends the run.
- Journaled "rounding": "cumulative". The per-close rule is kept as
  CloseTo::MembersPerClose for closes journaled "largest_remainder";
  older closes still replay with the last member absorbing it.
- Lean: runPart_telescopes (a member's total from a run is their
  allocation of the run's total) and closePart_total (each close hands
  out exactly its net), audited to the standard axioms.
- Tests: hand-computed three one-cent months (per close A 0.03 / B 0;
  cumulative A 0.02 / B 0.01), a run ending, a property that each member
  stays within a cent of their share (fails on the per-close rule), and
  a store contract that each rule journals and replays as itself.

Wanderland's books have no closes yet, so all of theirs use the new rule.

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

crates/ledger-core/src/ledger.rs

@@ -1294,6 +1294,8 @@ impl Ledger {
                 end,
                 status: PeriodStatus::Open,
                 closing_entry: None,
+                close_seq: None,
+                member_close: None,
             },
         );
         Ok(id)
@@ -1417,13 +1419,23 @@ impl Ledger {
                     sweep(*a, *into, self);
                 }
             }
-            periods::CloseTo::Members(members) | periods::CloseTo::MembersLastAbsorbs(members) => {
+            periods::CloseTo::Members(members)
+            | periods::CloseTo::MembersPerClose(members)
+            | periods::CloseTo::MembersLastAbsorbs(members) => {
                 let shares: Vec<Decimal> = members.iter().map(|m| m.share).collect();
                 let parts = match to {
                     periods::CloseTo::MembersLastAbsorbs(_) => {
                         periods::allocate_last_absorbs(net, &shares)
                     }
-                    _ => periods::allocate(net, &shares),
+                    periods::CloseTo::MembersPerClose(_) => periods::allocate(net, &shares),
+                    _ => {
+                        // Cumulative: what the run gives each member with
+                        // this close, less what it gave them before.
+                        let before = self.member_close_run(&member_key(members));
+                        let after = periods::allocate(before + net, &shares);
+                        let prior = periods::allocate(before, &shares);
+                        after.iter().zip(&prior).map(|(a, p)| a - p).collect()
+                    }
                 };
                 for (m, part) in members.iter().zip(parts) {
                     equity.push((m.capital, part));
@@ -1462,12 +1474,46 @@ impl Ledger {
             Some(entry)
         };
 
+        let close_seq = self
+            .periods
+            .values()
+            .filter(|p| p.status == PeriodStatus::Closed)
+            .count() as u64
+            + 1;
         let p = self.periods.get_mut(&id).expect("period exists");
         p.status = PeriodStatus::Closed;
         p.closing_entry = closing_entry;
+        p.close_seq = Some(close_seq);
+        p.member_close = match to {
+            periods::CloseTo::Members(members) => Some(periods::MemberCloseRecord {
+                members: member_key(members),
+                net,
+            }),
+            _ => None,
+        };
         Ok(closing_entry)
     }
 
+    /// The net closed by the run of cumulative members closes, with exactly
+    /// these members and shares, that ends with the latest close: zero if
+    /// the latest close was anything else (or there is none).
+    fn member_close_run(&self, members: &[(AccountId, Decimal)]) -> Decimal {
+        let mut closed: Vec<&periods::Period> = self
+            .periods
+            .values()
+            .filter(|p| p.status == PeriodStatus::Closed)
+            .collect();
+        closed.sort_by_key(|p| std::cmp::Reverse(p.close_seq));
+        let mut total = Decimal::ZERO;
+        for p in closed {
+            match &p.member_close {
+                Some(r) if r.members == members => total += r.net,
+                _ => break,
+            }
+        }
+        total
+    }
+
     /// Revenue and expense balances as of `end` as closing lines (code
     /// order), and the net income they carry (revenue debits minus expense
     /// credits).
@@ -1519,7 +1565,9 @@ impl Ledger {
                 accounts.push(*into);
                 accounts.extend(sweep);
             }
-            periods::CloseTo::Members(members) | periods::CloseTo::MembersLastAbsorbs(members) => {
+            periods::CloseTo::Members(members)
+            | periods::CloseTo::MembersPerClose(members)
+            | periods::CloseTo::MembersLastAbsorbs(members) => {
                 let total: Decimal = members.iter().map(|m| m.share).sum();
                 if members.is_empty()
                     || total != Decimal::ONE
@@ -1623,7 +1671,9 @@ impl Ledger {
         let sweeps: Vec<AccountId> = match to {
             periods::CloseTo::RetainedEarnings { sweep }
             | periods::CloseTo::Account { sweep, .. } => sweep.clone(),
-            periods::CloseTo::Members(members) | periods::CloseTo::MembersLastAbsorbs(members) => {
+            periods::CloseTo::Members(members)
+            | periods::CloseTo::MembersPerClose(members)
+            | periods::CloseTo::MembersLastAbsorbs(members) => {
                 members.iter().filter_map(|m| m.distributions).collect()
             }
         };
@@ -2770,3 +2820,9 @@ fn check_minor_units(amount: Money) -> Result<(), LedgerError> {
         _ => Ok(()),
     }
 }
+
+/// A members close's (capital account, share) list, in order: what a run
+/// of cumulative closes must share.
+fn member_key(members: &[periods::MemberClose]) -> Vec<(AccountId, Decimal)> {
+    members.iter().map(|m| (m.capital, m.share)).collect()
+}

crates/ledger-core/src/lib.rs

@@ -57,7 +57,9 @@ pub use ledger::{
 pub use money::{Currency, Money};
 pub use payouts::{Payout, PayoutAdjustment, PayoutLine, PayoutTotals, payout_plan};
 pub use payroll::{PayrollLine, PayrollRun, PayrollRunId};
-pub use periods::{CloseTo, MemberClose, Period, PeriodId, PeriodStatus, RETAINED_EARNINGS_CODE};
+pub use periods::{
+    CloseTo, MemberClose, MemberCloseRecord, Period, PeriodId, PeriodStatus, RETAINED_EARNINGS_CODE,
+};
 pub use reports::{
     BalanceSheet, BalanceSheetStatement, CashFlowLine, CashFlowParams, CashFlowStatement,
     ClassProfitAndLoss, GeneralLedger, LedgerAccount, LedgerPosting, MemberCapital,

crates/ledger-core/src/periods.rs

@@ -51,8 +51,15 @@ pub enum CloseTo {
     /// Members' capital accounts by share (shares sum to 1): the
     /// partnership convention, where each member's capital account carries
     /// their allocated earnings and absorbs their distributions. Odd cents
-    /// go by largest remainder ([`allocate`]).
+    /// go by largest remainder ([`allocate`]), cumulatively: a run of
+    /// member closes with the same members and shares gives each member,
+    /// in total, their share of the run's total to the cent, so rounding
+    /// never piles up close after close (rules.md B-13; since 2026-10-13).
     Members(Vec<MemberClose>),
+    /// [`CloseTo::Members`] rounding each close on its own (largest
+    /// remainder), the rule for closes journaled on 2026-10-13 before the
+    /// cumulative one. Only replay of those journals produces it.
+    MembersPerClose(Vec<MemberClose>),
     /// [`CloseTo::Members`] under the rounding closes used before
     /// 2026-10-13 ([`allocate_last_absorbs`]). Only replay of those journals
     /// produces it; new closes use `Members`.
@@ -134,6 +141,23 @@ pub struct Period {
     pub status: PeriodStatus,
     /// The closing entry's id, once closed.
     pub closing_entry: Option<u64>,
+    /// The order it closed in among all closes (1 for the first), once
+    /// closed.
+    #[serde(default)]
+    pub close_seq: Option<u64>,
+    /// What a cumulative members close allocated, once closed that way.
+    #[serde(default)]
+    pub member_close: Option<MemberCloseRecord>,
+}
+
+/// A cumulative members close ([`CloseTo::Members`]): who shared, and the
+/// net it closed. The next close with the same members continues the run.
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct MemberCloseRecord {
+    /// (capital account, share), in the order given.
+    pub members: Vec<(crate::AccountId, rust_decimal::Decimal)>,
+    /// The net income this close allocated (negative for a loss).
+    pub net: rust_decimal::Decimal,
 }
 
 impl Period {

crates/ledger-core/tests/statements.rs

@@ -1522,3 +1522,177 @@ mod chaining {
         }
     }
 }
+
+mod cumulative_closes {
+    use super::*;
+    use ledger_core::{CloseTo, MemberClose};
+
+    /// Three months of one-cent income, 50/50, each month closed into the
+    /// members' capital.
+    fn three_cent_months(to: impl Fn(&[AccountId; 2]) -> CloseTo) -> (Ledger, [AccountId; 2]) {
+        let mut l = Ledger::new(Currency::USD);
+        let mut add = |code: &str, t| l.add_account(code, code, t, None, false).unwrap();
+        let cash = add("1000", AccountType::Asset);
+        let caps = [
+            add("3000", AccountType::Equity),
+            add("3001", AccountType::Equity),
+        ];
+        let rev = add("4000", AccountType::Revenue);
+        for m in 1..=3 {
+            let e = l.create_entry(d(m, 10), "a cent");
+            l.add_line(
+                e,
+                cash,
+                Side::Debit,
+                Money::new(cents("0.01"), Currency::USD),
+            )
+            .unwrap();
+            l.add_line(
+                e,
+                rev,
+                Side::Credit,
+                Money::new(cents("0.01"), Currency::USD),
+            )
+            .unwrap();
+            l.post_entry(e).unwrap();
+            let end = d(m + 1, 1).pred_opt().unwrap();
+            let p = l
+                .create_period(&format!("2026-{m:02}"), d(m, 1), end)
+                .unwrap();
+            l.close_period_to(p, &to(&caps)).unwrap();
+        }
+        (l, caps)
+    }
+
+    fn half(caps: &[AccountId; 2]) -> Vec<MemberClose> {
+        caps.iter()
+            .map(|&capital| MemberClose {
+                capital,
+                share: cents("0.5"),
+                distributions: None,
+            })
+            .collect()
+    }
+
+    /// Oracle: rules.md B-13 (cumulative closes; docs/testing.md #12),
+    /// hand-computed. Closing each cent on its own gives the earlier member
+    /// every tie: A 0.03, B 0.00 after three months. Cumulatively: after
+    /// month 1 the run's 0.01 splits A 0.01 / B 0; after month 2, 0.02
+    /// splits 0.01 / 0.01, so month 2 gives B the cent; after month 3, 0.03
+    /// splits 0.02 / 0.01, so month 3 gives A the cent. A 0.02, B 0.01.
+    #[test]
+    fn a_run_of_closes_never_piles_up_rounding() {
+        let (l, caps) = three_cent_months(|c| CloseTo::MembersPerClose(half(c)));
+        let bal = |l: &Ledger, a| l.account_balance(a).unwrap();
+        assert_eq!(
+            (bal(&l, caps[0]), bal(&l, caps[1])),
+            (cents("0.03"), cents("0.00")),
+            "per close: the earlier member takes every tie"
+        );
+        let (l, caps) = three_cent_months(|c| CloseTo::Members(half(c)));
+        assert_eq!(
+            (bal(&l, caps[0]), bal(&l, caps[1])),
+            (cents("0.02"), cents("0.01")),
+            "cumulative: within a cent of half of 0.03 each"
+        );
+    }
+
+    proptest::proptest! {
+        /// Oracle: rules.md B-13, cumulative closes. For any run of monthly
+        /// profits and losses closed into three members at 33.33% /
+        /// 33.33% / 33.34%, each member's total from the run is within one
+        /// cent of their exact share of everything closed.
+        #[test]
+        fn each_member_stays_within_a_cent_of_their_share(
+            nets in proptest::collection::vec(-500_000i64..500_000, 1..12),
+        ) {
+            let mut l = Ledger::new(Currency::USD);
+            let mut add = |code: &str, t| l.add_account(code, code, t, None, false).unwrap();
+            let cash = add("1000", AccountType::Asset);
+            let caps = [
+                add("3000", AccountType::Equity),
+                add("3001", AccountType::Equity),
+                add("3002", AccountType::Equity),
+            ];
+            let rev = add("4000", AccountType::Revenue);
+            let exp = add("6000", AccountType::Expense);
+            let shares = [cents("0.3333"), cents("0.3333"), cents("0.3334")];
+            let to = CloseTo::Members(
+                caps.iter()
+                    .zip(shares)
+                    .map(|(&capital, share)| MemberClose { capital, share, distributions: None })
+                    .collect(),
+            );
+            for (i, n) in nets.iter().enumerate() {
+                let m = i as u32 + 1;
+                if *n != 0 {
+                    let amount = Money::new(Decimal::new(n.abs(), 2), Currency::USD);
+                    let (dr, cr) = if *n > 0 { (cash, rev) } else { (exp, cash) };
+                    let e = l.create_entry(d(m, 10), "month");
+                    l.add_line(e, dr, Side::Debit, amount).unwrap();
+                    l.add_line(e, cr, Side::Credit, amount).unwrap();
+                    l.post_entry(e).unwrap();
+                }
+                let end = if m == 12 { d(12, 31) } else { d(m + 1, 1).pred_opt().unwrap() };
+                let p = l.create_period(&format!("2026-{m:02}"), d(m, 1), end).unwrap();
+                l.close_period_to(p, &to).unwrap();
+            }
+            let total = Decimal::new(nets.iter().sum::<i64>(), 2);
+            for (cap, share) in caps.iter().zip(shares) {
+                let got = l.account_balance(*cap).unwrap();
+                proptest::prop_assert!((got - total * share).abs() < cents("0.01"), "{got} vs {}", total * share);
+            }
+        }
+    }
+
+    /// A run is closes with the same members and shares, back to back: a
+    /// close to anything else (or with other shares) ends it, and the next
+    /// members close starts afresh.
+    #[test]
+    fn a_different_close_ends_the_run() {
+        let mut l = Ledger::new(Currency::USD);
+        let mut add = |code: &str, t| l.add_account(code, code, t, None, false).unwrap();
+        let cash = add("1000", AccountType::Asset);
+        let caps = [
+            add("3000", AccountType::Equity),
+            add("3001", AccountType::Equity),
+        ];
+        let rev = add("4000", AccountType::Revenue);
+        let cent = |l: &mut Ledger, m| {
+            let e = l.create_entry(d(m, 10), "a cent");
+            l.add_line(
+                e,
+                cash,
+                Side::Debit,
+                Money::new(cents("0.01"), Currency::USD),
+            )
+            .unwrap();
+            l.add_line(
+                e,
+                rev,
+                Side::Credit,
+                Money::new(cents("0.01"), Currency::USD),
+            )
+            .unwrap();
+            l.post_entry(e).unwrap();
+        };
+        let close = |l: &mut Ledger, m: u32, to: &CloseTo| {
+            let end = d(m + 1, 1).pred_opt().unwrap();
+            let p = l
+                .create_period(&format!("2026-{m:02}"), d(m, 1), end)
+                .unwrap();
+            l.close_period_to(p, to).unwrap();
+        };
+        // Month 1 to members (A gets the cent), month 2 to retained
+        // earnings (ends the run), month 3 to members again: a new run, so
+        // A gets that cent too.
+        cent(&mut l, 1);
+        close(&mut l, 1, &CloseTo::Members(half(&caps)));
+        cent(&mut l, 2);
+        close(&mut l, 2, &CloseTo::RetainedEarnings { sweep: vec![] });
+        cent(&mut l, 3);
+        close(&mut l, 3, &CloseTo::Members(half(&caps)));
+        assert_eq!(l.account_balance(caps[0]).unwrap(), cents("0.02"));
+        assert_eq!(l.account_balance(caps[1]).unwrap(), cents("0"));
+    }
+}

crates/ledger-store/src/store.rs

@@ -1099,7 +1099,9 @@ impl Store {
             CloseTo::RetainedEarnings { sweep } if sweep.is_empty() => json!("retained_earnings"),
             CloseTo::RetainedEarnings { sweep } => json!({"retained_earnings": {"sweep": sweep}}),
             CloseTo::Account { into, sweep } => json!({"account": {"into": into, "sweep": sweep}}),
-            CloseTo::Members(members) | CloseTo::MembersLastAbsorbs(members) => {
+            CloseTo::Members(members)
+            | CloseTo::MembersPerClose(members)
+            | CloseTo::MembersLastAbsorbs(members) => {
                 let mut v = json!({
                     "members": members
                         .iter()
@@ -1110,10 +1112,13 @@ impl Store {
                         }))
                         .collect::<Vec<_>>(),
                 });
-                // Closes journaled before 2026-10-13 carry no rounding and
-                // replay with the last member absorbing it.
-                if matches!(to, CloseTo::Members(_)) {
-                    v["rounding"] = json!("largest_remainder");
+                // The rounding rule, so replay closes the same way. Closes
+                // journaled before 2026-10-13 carry none (the last member
+                // absorbed it).
+                match to {
+                    CloseTo::Members(_) => v["rounding"] = json!("cumulative"),
+                    CloseTo::MembersPerClose(_) => v["rounding"] = json!("largest_remainder"),
+                    _ => {}
                 }
                 v
             }
@@ -3487,10 +3492,12 @@ fn close_to_from_journal(v: Option<&Value>) -> Result<CloseTo, StoreError> {
         })
         .collect::<Result<_, StoreError>>()?;
     // The rounding rule the close was made under: absent before
-    // 2026-10-13, when the last member absorbed it.
+    // 2026-10-13, when the last member absorbed it; "largest_remainder"
+    // per close, then "cumulative" over a run of closes.
     match v.get("rounding").and_then(|r| r.as_str()) {
         None => Ok(CloseTo::MembersLastAbsorbs(members)),
-        Some("largest_remainder") => Ok(CloseTo::Members(members)),
+        Some("largest_remainder") => Ok(CloseTo::MembersPerClose(members)),
+        Some("cumulative") => Ok(CloseTo::Members(members)),
         Some(_) => Err(corrupt("unknown rounding")),
     }
 }

crates/ledger-store/tests/store.rs

@@ -1698,3 +1698,69 @@ fn scrapping_an_asset_for_nothing_disposes_it() {
 fn store_add(store: &mut Store, code: &str, t: AccountType) -> ledger_core::AccountId {
     store.add_account(code, code, t, None, false).unwrap()
 }
+
+/// Contract (rules.md B-13): each members close journals its rounding rule
+/// and replays under it: "cumulative" for new closes, "largest_remainder"
+/// for the per-close rule of earlier on 2026-10-13. Three one-cent months,
+/// 50/50: cumulative ends A 0.02 / B 0.01, per close A 0.03 / B 0.00.
+#[test]
+fn members_closes_replay_under_the_rule_they_were_journaled_with() {
+    for (cumulative, rule, a, b) in [
+        (true, "\"rounding\":\"cumulative\"", "0.02", "0.01"),
+        (false, "\"rounding\":\"largest_remainder\"", "0.03", "0.00"),
+    ] {
+        let dir = test_dir(&format!("close-rule-{cumulative}"));
+        let mut store = Store::create(&dir, Currency::USD).unwrap();
+        let cash = store
+            .add_account("1000", "Cash", AccountType::Asset, None, false)
+            .unwrap();
+        let caps = [
+            store
+                .add_account("3000", "A", AccountType::Equity, None, false)
+                .unwrap(),
+            store
+                .add_account("3001", "B", AccountType::Equity, None, false)
+                .unwrap(),
+        ];
+        let rev = store
+            .add_account("4000", "Sales", AccountType::Revenue, None, false)
+            .unwrap();
+        let members: Vec<ledger_core::MemberClose> = caps
+            .iter()
+            .map(|&capital| ledger_core::MemberClose {
+                capital,
+                share: Decimal::new(5, 1),
+                distributions: None,
+            })
+            .collect();
+        let to = if cumulative {
+            ledger_core::CloseTo::Members(members)
+        } else {
+            ledger_core::CloseTo::MembersPerClose(members)
+        };
+        let cent = Money::new(Decimal::new(1, 2), Currency::USD);
+        for m in 1..=3u32 {
+            let day = |d| NaiveDate::from_ymd_opt(2026, m, d).unwrap();
+            let e = store.create_entry(day(10), "a cent").unwrap();
+            store.add_line(e, cash, Side::Debit, cent).unwrap();
+            store.add_line(e, rev, Side::Credit, cent).unwrap();
+            store.post_entry(e).unwrap();
+            let end = NaiveDate::from_ymd_opt(2026, m + 1, 1)
+                .unwrap()
+                .pred_opt()
+                .unwrap();
+            let p = store
+                .create_period(&format!("2026-{m:02}"), day(1), end)
+                .unwrap();
+            store.close_period_to(p, &to).unwrap();
+        }
+        let journal = std::fs::read_to_string(dir.join(ledger_store::JOURNAL_FILE)).unwrap();
+        assert_eq!(journal.matches(rule).count(), 3, "{rule}");
+        drop(store);
+        let store = Store::open(&dir).unwrap();
+        store.verify().unwrap();
+        let bal = |id| store.ledger().account_balance(id).unwrap();
+        assert_eq!(bal(caps[0]), a.parse::<Decimal>().unwrap(), "{rule}");
+        assert_eq!(bal(caps[1]), b.parse::<Decimal>().unwrap(), "{rule}");
+    }
+}

docs/spec/rules.md

@@ -217,6 +217,14 @@ closed period).
 - **No authority** says how to round a share to the cent. Largest
   remainder, ties to the earlier member: DECISION (Stephen, 2026-10-13).
   Closes made before then replay with the last member absorbing it.
+- Rounding never piles up: a run of members closes (same members and
+  shares, back to back) splits cumulatively, each close giving a member
+  their allocation of the run's total so far less their allocation before
+  it, so each member's total from the run is within a cent of their exact
+  share (follows from the running capital account below; found
+  2026-10-13: closing each month on its own gave every tied cent to the
+  earlier member). Journaled as `"rounding": "cumulative"`; closes
+  journaled `"largest_remainder"` replay one at a time.
 - A capital account is one running balance: Form 1065 Schedule K-1
   instructions, Item L: "The partnership must report your beginning
   capital account and ending capital account for the year", "based on the
@@ -229,8 +237,12 @@ closed period).
 
 **Enforced:** `CloseTo::Members`, `periods::allocate`,
 `kernel::apportion`, `members_equity`. **Checked:** Kani
-`apportion_is_exact`, `apportion_is_fair`; properties `prop_split_*`,
-`each_members_capital_chains_across_statements`;
+`apportion_is_exact`, `apportion_is_fair`; Lean `runPart_telescopes`,
+`closePart_total`; properties `prop_split_*`,
+`each_members_capital_chains_across_statements`,
+`each_member_stays_within_a_cent_of_their_share`;
+`a_run_of_closes_never_piles_up_rounding`,
+`members_closes_replay_under_the_rule_they_were_journaled_with`;
 `closes_journaled_under_the_old_rule_replay_with_it`.
 
 ## B-14 Statements: P&L for a period; cash flow by the indirect method

docs/testing.md

@@ -79,7 +79,7 @@ accounting question), CHANGE (a source says otherwise; work pending).
 | 9 | An asset's life ends at purchase + life months, that day not held; day-count proration | CONVENTION | Exactly N months of service; day proration and the mid-month convention are both accepted book policies (ASC 360-10-35-4 not read: login) |
 | 10 | A payroll entry has one line per account total | CONFIRMED | The per-employee detail the IRS requires ("Amounts and dates of all wage ... payments", Employment Tax Recordkeeping) is kept in the run; a summary entry is standard |
 | 11 | A period with exactly the dates of an existing one is allowed | DESIGN | Nothing governs duplicate periods; harmless (the second close locks only). Refusing exact duplicates is an option |
-| 12 | Each close into members' capital rounds on its own (ties to the earlier member) | CHANGE (pending) | Found 2026-10-13 with #7: twelve monthly 50/50 closes can leave one member several cents ahead of half the year's income. Closes should split cumulatively too (year-to-date allocation minus what earlier closes gave); a new journaled rounding rule, older closes replaying as booked |
+| 12 | Each close into members' capital rounds on its own (ties to the earlier member) | CHANGED 2026-10-13 | Twelve monthly 50/50 closes could leave one member several cents ahead. A run of members closes now splits cumulatively (rules.md B-13; Lean `runPart_telescopes`; property `each_member_stays_within_a_cent_of_their_share` fails on the per-close rule); journaled `"cumulative"`, older closes replay as booked |
 
 Questions are settled from the sources, not by any one company's CPA
 (wanderbooks is general software). Where the standards leave a real

docs/verification.md

@@ -124,6 +124,13 @@ sequence of commands, of any length**:
     other shares were rounded, and only the last share changes.
     - Kani `plug_last_sums_exactly` · Lean `plugLast_total`,
       `plugLast_keeps`
+17. **A run of member closes telescopes**: each member's total from a run
+    of cumulative closes is their allocation of everything the run closed
+    (so its rounding is one allocation's, never a sum of them), and each
+    close hands out exactly its net, for any allocation whose parts sum to
+    the whole (rules.md B-13).
+    - Lean `runPart_telescopes`, `closePart_total` · property
+      `each_member_stays_within_a_cent_of_their_share`
 
 ## What the proofs found
 

verify/lean/Audit.lean

@@ -19,6 +19,8 @@ open Wanderbooks
 #print axioms net_closings_is_net_income
 #print axioms plugLast_total
 #print axioms plugLast_keeps
+#print axioms runPart_telescopes
+#print axioms closePart_total
 #print axioms Reachable.inv
 #print axioms Reachable.books_net_zero
 #print axioms Reachable.trial_balance

verify/lean/Wanderbooks/Closing.lean

@@ -317,4 +317,54 @@ theorem plugLast_keeps (whole : Int) (ps : List Int) :
       rw [show plugLast whole (p :: q :: rest) = p :: plugLast (whole - p) (q :: rest) from rfl,
         List.dropLast_cons_of_ne_nil hne, ih, List.dropLast_cons_cons]
 
+/-! ## Cumulative member closes
+
+A run of members closes (`CloseTo::Members`, rules.md B-13) gives each
+member, at each close, their allocation of the run's total so far minus
+their allocation of the total before it. `alloc` is any allocation of an
+amount to members (member `i` gets `alloc x i`), such as
+`periods::allocate`. -/
+
+/-- What one close gives member `i`: `before` is the run's total before
+it, `net` what it closes. -/
+def closePart (alloc : Int → Nat → Int) (before net : Int) (i : Nat) : Int :=
+  alloc (before + net) i - alloc before i
+
+/-- What a run of closes with these nets, starting after `before`, gives
+member `i` in all. -/
+def runPart (alloc : Int → Nat → Int) : Int → List Int → Nat → Int
+  | _, [], _ => 0
+  | before, net :: rest, i => closePart alloc before net i + runPart alloc (before + net) rest i
+
+/-- **A run telescopes**: member `i`'s total from a run is their allocation
+of everything the run closed, less their allocation of where it started.
+From an empty start (`alloc 0 = 0`), it is exactly their allocation of
+the run's total, so its rounding is one allocation's rounding, never a
+sum of them. -/
+theorem runPart_telescopes (alloc : Int → Nat → Int) (before : Int) (nets : List Int) (i : Nat) :
+    runPart alloc before nets i = alloc (before + nets.sum) i - alloc before i := by
+  induction nets generalizing before with
+  | nil => simp [runPart]
+  | cons n rest ih =>
+    simp only [runPart, closePart, ih, List.sum_cons]
+    rw [show before + n + rest.sum = before + (n + rest.sum) by omega]
+    omega
+
+/-- A sum of differences is the difference of the sums. -/
+theorem sum_map_sub (f g : Nat → Int) (ms : List Nat) :
+    (ms.map (fun i => f i - g i)).sum = (ms.map f).sum - (ms.map g).sum := by
+  induction ms with
+  | nil => simp
+  | cons m ms ih => simp only [List.map_cons, List.sum_cons, ih]; omega
+
+/-- **Each close hands out exactly its net** when every allocation sums
+to its amount (`periods::allocate` does: `kernel::apportion`). -/
+theorem closePart_total (alloc : Int → Nat → Int) (members : List Nat)
+    (sums : ∀ x, (members.map (alloc x)).sum = x) (before net : Int) :
+    (members.map (closePart alloc before net)).sum = net := by
+  have h := sum_map_sub (alloc (before + net)) (alloc before) members
+  unfold closePart
+  rw [h, sums, sums]
+  omega
+
 end Wanderbooks