wanderbooks/docs/spec/rules.md

Rule catalog

Every rule the engine enforces, the authority for it, and what checks it. Status is AUTHORITY (a source states the rule), CONVENTION (common practice no authority states; the best reference is given), or DECISION (a human chose, and when). Vendored sources are in sources/; FASB and OpenStax are quoted briefly under their copyright and license (see README.md). Quotes from FASB's Codification were read in Deloitte's DART roadmaps, which reproduce it ("FASB Accounting Standards Codification: Copyright © 2005 - 2026 by Financial Accounting Foundation").

Spec blocks in the code cite these as SOURCE: rules.md B-n.

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B-1 Every entry is a double entry: debits equal credits

Status: AUTHORITY

• Pacioli ch. 36 (sources/pacioli-1494.md): "All entries made in the ledger have to be double entries — that is, if you make one creditor, you must make some one debtor."
• IRS Pub. 583 (sources/irs-records.md): "It is self-balancing because you record every transaction as a debit entry in one account and as a credit entry in another. Under this system, the total debits must equal the 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). A line's account must exist and take postings when the line is added, not only at post: DECISION (Stephen, 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.

B-2 A line's amount is positive; its side gives the direction

Status: CONVENTION

Pacioli keeps amounts in a debit column and a credit column (ch. 36: "All the creditors must appear in the Ledger at the right hand side, and all the debtors at the left"); the side, not a sign, says which way money moves. No authority forbids a negative amount outright; wanderbooks refuses them so that every line reads one way (2026-10-13, found by the formal pass).

Enforced: kernel::check_lines (NegativeAmountLine, ZeroAmountLine). Checked: Kani check_lines_is_exact; test negative_amount_line_is_rejected.

B-3 One money through the ledger; foreign amounts at the transaction-date rate

Status: AUTHORITY

• Pacioli ch. 36: "the money that you reckon by at the beginning should be the same all through the Ledger."
• ASC 830-20-30-1 (cited): "each asset, liability, revenue, expense, gain, or loss arising from the transaction shall be measured initially in the functional currency of the recording entity by use of the exchange rate in effect at that date."

A foreign line is booked in the base currency's smallest unit (ISO 4217 minor units, sources/iso-4217.md), half away from zero; a ledger in one money holds no fractions of its smallest unit, and its cash can then match a bank statement: DECISION (Stephen, 2026-10-13), after the GnuCash cross-check found wanderbooks kept $108.34567 where GnuCash booked $108.35. Lines booked before then replay as booked.

Enforced: Ledger::add_line (base currency only), Ledger::add_line_fx (base amount = foreign amount × rate, rounded to the minor unit; original kept). Checked: a_foreign_line_is_booked_in_whole_base_units; the GnuCash oracle. Checked: foreign_currency_line_is_rejected, fx_overflow_is_an_error.

B-4 Five elements, closed list

Status: AUTHORITY (definitions); the five-type chart is CONVENTION

FASB Concepts Statement No. 8, Ch. 4 (2021), which replaced Concepts Statement 6 (cited): E16 "An asset is a present right of an entity to an economic benefit." E37 "A liability is a present obligation of an entity to transfer an economic benefit." E61 equity is "the residual interest in the assets of an entity that remains after deducting its liabilities." E80 revenues "are inflows or other enhancements of assets … or settlements of 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). A child account has its parent's type: DECISION (Stephen, 2026-10-13), Ledger::add_account (ParentTypeMismatch). Checked: a_child_account_has_its_parents_type.

B-5 Normal balances

Status: CONVENTION

No standard defines normal balances. OpenStax, Principles of Accounting, Vol. 1 §3.1 (cited; "Access for free at openstax.org"): "As assets and expenses increase on the debit side, their normal balance is a debit… Since liabilities, equity (such as common stock), and revenues increase with a credit, their 'normal' balance is a credit." Pacioli ch. 12: capital "must always be placed as creditor … and the cash always debtor."

Enforced: AccountType::normal_side, kernel::normal_balance. Checked: Kani normal_balance_is_signed, normal_side_increases; Lean balance_after_line.

B-6 The trial balance balances

Status: AUTHORITY

Pacioli ch. 36: "the total of the credits — I do not say creditors — should be equal to the total of the debits — I do not say debtors. If they were not equal there would be a mistake in the Ledger." IRS Pub. 583 (B-1).

Enforced: follows from B-1. Checked: Kani trial_balance_columns; Lean trial_balance_iff, Reachable.trial_balance, Reachable.trial_balance_through; properties.

B-7 The accounting equation

Status: AUTHORITY

CON 8 Ch. 4 ¶E62 (cited): equity "is the difference between the entity's assets and its liabilities." On a live book, income not yet closed is part of equity (B-11).

Checked: Kani accounting_equation; Lean accounting_equation, Reachable.accounting_equation[_through]; property prop_balance_sheet_balances.

B-8 Posted is forever: the book is written only forward

Status: AUTHORITY

• Pacioli ch. 36: "You should not write backward, but always forward — that is, go forward as the days go, which never come back. If you do otherwise, the book would be untrue."
• German Commercial Code (HGB) §239(3) (cited; official English translation): "An entry or a documented record may not be modified such that it is no longer possible to establish its original content."
• Rev. Proc. 98-25 (sources/irs-records.md): machine-sensible records must keep the audit trail.

Enforced: posted entries immutable; the journal is append-only and hash-chained. Checked: Lean Step.extends, Steps.extends; property prop_posted_entries_immutable; store journal_tamper_is_detected.

B-9 A mistake is corrected by an opposite entry, never erased

Status: AUTHORITY

Pacioli ch. 31 (the storno): "you shall make another entry opposite this one in the credit for the same amount … After you have made this correction it is just as if you had written nothing in the debit column." Ch. 36: "Mark with a cross or with an "H" that special entry, and then make an entry on the opposite side under the same account."

Enforced: Ledger::reverse_entry (same lines, sides flipped, linked by reversal_of). Checked: Kani reversal_mirrors, reversal_cancels; Lean accepted_reversal_iff, movement_with_reversal.

B-10 An entry is reversed at most once; a closing entry never

Status: DECISION (Stephen, 2026-10-13)

Follows from B-9 (a correction corrects once) and B-11 (a closed period stays closed). Enforced: Ledger::check_reversal. Checked: an_entry_is_reversed_at_most_once, a_closing_entry_is_never_reversed.

B-11 Revenue and expenses close into profit and loss, and that into capital

Status: AUTHORITY (mechanics); the account the net lands in depends on the entity

• Pacioli ch. 34: "you shall close this account into the capital account"; ch. 27: profit and loss "must then be transferred for its closing (saldo) into the capital account."
• OpenStax §5.1 (cited): "Temporary (nominal) accounts are accounts that are closed at the end of each accounting period." Corporations close into retained earnings; partnerships into each partner's capital (B-13).

Enforced: Ledger::close_period_to. Checked: Kani closing_line_zeroes, closing_entry_balances; Lean closingEntry_zeroes, closingEntry_accepted, close_zeroes, close_credits_net_income; a_close_sweeps_only_through_the_period_end.

B-12 A closed period takes no new postings; periods close in order

Status: CONVENTION

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). Periods nest (a month in a quarter in a year) and never partially overlap: DECISION (Stephen, 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).

B-13 Partners share income by the partnership agreement; capital accounts carry it

Status: AUTHORITY (shares); DECISION (cent rounding)

• 26 U.S.C. §704(a) (sources/irs-partnerships.md): "A partner's distributive share of income, gain, loss, deduction, or credit shall, except as otherwise provided in this chapter, be determined by the partnership agreement."
• Form 1065 Schedule M-2: "Show what caused changes during the tax year in the partners' tax-basis capital accounts."
• 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.

Enforced: CloseTo::Members, periods::allocate, kernel::apportion, members_equity. Checked: Kani apportion_is_exact, apportion_is_fair; properties prop_split_*; closes_journaled_under_the_old_rule_replay_with_it.

B-14 Statements: P&L for a period; cash flow by the indirect method

Status: AUTHORITY

ASC 230-10-45-28 (cited): an entity not using the direct method "shall determine and report the same amount for net cash flow from operating activities indirectly by adjusting net income … (the indirect or reconciliation method)." ASC 230-10-45-16: "All of the following are cash inflows from operating activities: Cash receipts from sales of goods or services…". A closing entry is not income or spending (B-11), so period statements exclude it.

Enforced: profit_and_loss_for_period, cash_flow. Checked: Lean statements_add_up; prop_period_pnl_partitions; cash_flow_reconciles_to_change_in_cash.

B-15 Depreciation: straight line, salvage never depreciated, partial months by days

Status: CONVENTION (book method); AUTHORITY for tax conventions, which wanderbooks does not apply

OpenStax §11.3 (cited): "Straight-line depreciation is a method of depreciation that evenly splits the depreciable amount across the useful life of the asset," with "(Cost – salvage value)/Useful life"; partial periods: "A common method is to allocate depreciation expense based on the number of months the asset is owned in a year." wanderbooks prorates partial months by days. ASC 360-10-35-4 not yet verified (see README). Tax conventions (half-year, mid-month, mid-quarter): IRS Pub. 946 (sources/irs-depreciation.md), for the CPA.

Enforced: assets::depreciation_schedule. Checked: schedule_sums_exactly_to_depreciable_amount, salvage_value_is_never_depreciated, prop_depreciation_schedule_exact.

B-16 Prepaid expenses and deferred revenue are recognized over the period benefited or earned

Status: AUTHORITY (revenue); CONVENTION (prepaids)

ASC 606-10-45-2 (cited): payment before transfer → "the entity shall present the contract as a contract liability"; 606-10-25-23: "An entity shall recognize revenue when (or as) the entity satisfies a performance obligation by transferring a promised good or service." Prepaids: OpenStax §4.2 (cited). Tax: the 12-month rule, Treas. Reg. §1.263(a)-4(f) (sources/irs-capitalization.md).

Enforced: schedules. Checked: a_prepaid_expense_is_expensed_month_by_month, deferred_revenue_is_earned_to_the_cent.

B-17 Platform payouts are recorded gross when the business is the principal

Status: AUTHORITY

ASC 606-10-55-37 (cited): "An entity is a principal if it controls the specified good or service before that good or service is transferred to a customer." 606-10-55-38: an agent "recognizes revenue in the amount of any fee or commission to which it expects to be entitled." Whether an app developer is principal for app-store sales is a judgment under 55-39; the CPA confirms.

Enforced: payouts::payout_plan. Checked: a_payout_is_grossed_up_to_revenue_and_fees.

B-18 Aging buckets: Current, 1–30, 31–60, 61–90, 90+ days past due

Status: CONVENTION (bucket edges); DECISION (due today is Current)

ASC 326-20-55-37/38 (cited) illustrates "one way" to age trade receivables with buckets "current … 1–30 days past due … 31–60 … 61–90 … more than 90 days past due"; it is illustrative, not a rule. The FFIEC Call Report instructions (bank regulators, not binding here): "The past due status … should be determined in accordance with its contractual repayment terms." A document due today is not yet past due: DECISION (Stephen, 2026-10-13).

Enforced: arap::aging_bucket. Checked: aging_boundaries, aging_is_exclusive_and_only_ages.

B-19 1099 reporting thresholds

Status: AUTHORITY (sources/irs-1099-thresholds.md)

$600 through 2025; $2,000 for payments made after Dec. 31, 2025 (26 U.S.C. §6041(a), §6041A(a)(2), as amended by Pub. L. 119-21 §70433); for calendar years after 2026, indexed to inflation and rounded to the nearest $100 (§6041(h)). Card payments are reported on Form 1099-K, not 1099-NEC/MISC. Attorneys' gross proceeds stay at $600 (not modeled).

Enforced: form1099::default_threshold, a dated rule (ledger_core::rules::THRESHOLD_1099, known through 2026, review from 2026-11-01). For a later year the engine refuses with RuleNotKnown rather than guess; the MCP digest and the viewer's "Needs attention" list flag the rule once it is due (DECISION, Stephen, 2026-10-13). Checked: an_unknown_year_threshold_is_never_guessed, dated_rules_go_stale_on_their_review_date, MCP stdio test.

B-20 Every transaction has supporting documents; records are kept

Status: AUTHORITY (sources/irs-records.md)

IRS Pub. 583: "In addition, you must keep supporting documents"; they "support the entries in your books and on your tax return"; keep records "until the period of limitations for that return runs out" (3 years generally; 7 for bad debts).

Enforced: the MCP receipt policy (not the engine; R-13).

B-21 Exact amounts in the books; rounding only on returns

Status: AUTHORITY (for where rounding happens)

Form 1065 instructions (sources/irs-partnerships.md): "If two or more amounts are added to figure the amount to enter on a line, include cents when adding the amounts and round off only the total." The books keep cents (and FX precision) exactly (R-2).

Enforced: Decimal money, Amount checked arithmetic, line range. Checked: Kani sums_are_exact; bridge tests.

B-22 Expense or capitalize (book policy, mirrors tax)

Status: AUTHORITY (sources/irs-capitalization.md)

Treas. Reg. §1.263(a)-1(f)(1)(ii)(D) sets $500 per invoice or item for a taxpayer without an applicable financial statement "or other amount as identified in published guidance"; Notice 2015-82 raised it to $2,500 for taxable years beginning on or after January 1, 2016. The 12-month rule: §1.263(a)-4(f)(1). The election is made yearly on the return.

Enforced: bookkeeper policy (books' AGENTS.md), not the engine.