GEP 10: annotate the taxes and transfers system with units (#1192)

[MImmesberger]

Closes #1192. Implements GEP 10 (#1193) across the German taxes and transfers system, and
restates every pre-2002 currency parameter in Deutsche Mark. (#1211 is merged into this
branch; there is one PR, not a stack.)

Every @policy_function, @policy_input, @param_function and
@group_creation_function now declares a unit, as does every parameter in the YAML files.
The engine checks those declarations against what the function bodies actually do, so a
column that claims to be a monthly amount cannot quietly be built from a yearly one. The
euro is the base currency, the Deutsche Mark is registered relative to it, and policy
dates before 2002 compute in the currency of their day.

Why this is one large PR

unit= is a required argument of @policy_function. gettsim cannot import until every
decorator carries one, so the migration is atomic: there is no intermediate state that
runs. The commits are split per namespace to keep the diff readable, but only the final
state is green.

The work came in three passes.

Annotation — one commit per namespace, declaring the unit of every function, input and
parameter.

Round C — making the bodies agree with the declarations. This is where the interesting
failures were: literals needing a cast_unit to say which quantity a bare number is,
comparisons across levels, counts multiplying amounts, and places where the declared unit
turned out to be the wrong one.

Deutsche Mark — the statutory currency is the Deutsche Mark through 2001 and the euro
from 2002, and the pre-2002 parameters are restated accordingly.

Renames

Where a name did not match the quantity, the name changed. Most columns gain the suffix of
the period they are measured over (basistarif → basistarif_y, rentenwert →
rentenwert_m). Two lose one they should never have had, being shares rather than flows:
anteil_steuerfälliger_einnahmen_y and mehrbedarf_alleinerziehend_m.
gesamteinkommen_y becomes gesamteinkommen_y_sn, naming the level it aggregates to.

These are breaking and ship without aliases, so the changelog lists all 22 by name.

The Deutsche-Mark values were read from the Bundesgesetzblatt

Not obtained by multiplying the stored euro values by 1,95583. That matters more than it
sounds, because the stored values are lossy and inverting them is ambiguous or wrong:

• The Rentenwert (Ost) for 1994 is 33,34 DM. The stored 17,05 EUR is equally
  consistent with 33,35; only § 2 V. v. 01.12.1993 settles it.
• The Arbeitnehmerpauschbetrag stored 1 044 EUR from 1990, but that is 2 042 DM, and the
  law read 2 000 DM until 2001. Inverting would have enshrined 2 042.

Where the law states a monthly amount and the parameter is yearly (§ 32 Abs. 6 EStG's
"261 Deutsche Mark", § 15 WoGG's "1 200 Deutsche Mark im Jahr"), the entries say so.

Corrections found by reading the gazette

Fourteen pre-existing errors surfaced. The ones that change results:

parameter	was	gazette
Wohngeld Höchstbetrag, 4 Mitglieder, Baujahr ≤1965, Mietstufe V	585 EUR	485 (948,58 DM) — broke Mietstufe monotonicity, overstating the cap by 100 EUR/month
same, 3 Mitglieder, Mietstufe I	322 EUR	320 (625,87 DM)
Wohngeld Mehrbetrag 1985, Baujahr 1972–1977	copy of the 1966–1971 column	85/90/95/100/105 DM
Wohngeld Mehrbetrag 1990-10, Baujahr ≥1978	near-copy of the 1966–1977 column	95/105/110/120/125/135 DM
Wohngeld 1993, 4 Mitglieder, Mietstufe VI	437 EUR (a duplicate of Mietstufe II)	578 (1 130 DM)
Rentenversicherung-Grenze 1999 West	4 339 EUR (= 8 486 DM)	8 500 DM = 4 346 EUR
Bezugsgröße Ost 1998	3 710 DM (the 1999 value)	3 640 DM
Wohngeld Behinderten-Freibetrag 1986	halved to 612 EUR	unchanged at 2 400 DM — the parameter's own description had suspected this
Kinderfreibetrag, Betreuung 2000	774 EUR	1 512 DM

Three parameters held the next law's value a year early: arbeitnehmerpauschbetrag,
maximales_einkommen_des_kindes (14 040 DM applies "ab dem Veranlagungszeitraum 2002"),
and the Wohngeld Höchstbeträge, whose 2001 entry held the Artikel-6 euro table. The
Haushaltsfreibetrag's 2 916 EUR is likewise the changeover value: the
Steuer-Euroglättungsgesetz glättet § 32 "durch 54 teilbare" Euro-Beträge, and 5 616 DM
lands on 54 × 54.

Three cross-checks did the catching, and are worth keeping in mind for future work: § 8
SGB IV puts the Minijobgrenze at a seventh of the Bezugsgröße, § 6 Abs. 7 SGB V puts the
Krankenversicherung grenze at three quarters of the Rentenversicherung one, and the
Wohngeld tables rise monotonically in the Mietstufe.

Mid-year changes and three values that are not in the Bundesgesetzblatt

The Ost Rechengrößen changed twice in 1991, so those entries are split at 1 July, as the
law is. The Minijobgrenze had held 235 — the average of 220 and 250, and the only year
that broke the seventh rule.

Three values are genuinely not gazetted, and each entry records why: the Beitrittsgebiet's
1990 and first-half-1991 Rechengrößen rest on the DDR's Gesetz über die
Sozialversicherung (GBl. I Nr. 38 S. 486); § 68 Abs. 1 SGB VI defines the first aktueller
Rentenwert as the December 1991 pension rather than an amount; and beitragsvolumen and
gesamtes_rentenvolumen are Deutsche-Rentenversicherung statistics that no function
reads, so their pre-2002 entries are dropped rather than converted.

mietobergrenze_pro_qm now starts in 2005: Arbeitslosengeld II does not exist earlier, and
the value is a rule of thumb that, as its description says, is not in the law.

Two further decisions worth knowing about

mean_entgeltpunkte_pro_bewertungsmonat is dimensionless — total Entgeltpunkte divided by
a plain count of months. It had carried _y, which pulled it through automatic time
conversion and changed the number. Dropping the suffix fixes the Erwerbsminderungsrente.

The Kinderzuschlag satz → satz_m rename had been applied to the param functions and
their consumers but not to the raw parameter, which silently left 2023 without a producer.
A missing root node prunes a whole subtree from the unit check, so it also masked a genuine
unit error further down.

Results

Only five cases move: the Erwerbsminderungsrente at 2001, by 0.006 to 0.013 percent. The
Rentenwert had been stored as 24,84 EUR, a rounded conversion, where the Verordnung gives
48,58 DM. Policy cases stay denominated in euro; the model computes in the statutory
currency and converts at the column boundary.

That so little moves is the point: a wrong Deutsche-Mark value would show up as a
policy-sized jump, not a fourth decimal.

Before merging

The Wohngeld typo fixes and the 1999 Grenze also change results for 2002 to 2008. They
are genuine corrections rather than redenomination.

This branch temporarily pins ttsim-backend to the GEP-10 ttsim branch so that CI installs
an engine with the units machinery. The pin reverts to main before this merges, once
ttsim-dev/ttsim#138 and #141 are in.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 97.26776% with 20 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/gettsim/germany/wohngeld/wohngeld.py	57.14%	6 Missing ⚠️
...cherung/rente/altersrente/hinzuverdienstgrenzen.py	50.00%	4 Missing ⚠️
.../sozialversicherung/rente/grundrente/grundrente.py	76.92%	1 Missing and 2 partials ⚠️
...many/einkommensteuer/zu_versteuerndes_einkommen.py	66.66%	2 Missing ⚠️
...ermany/sozialversicherung/rente/beitrag/beitrag.py	86.66%	2 Missing ⚠️
...many/sozialversicherung/kranken/beitrag/beitrag.py	90.90%	1 Missing ⚠️
...rmany/sozialversicherung/pflege/beitrag/beitrag.py	90.00%	1 Missing ⚠️
...ozialversicherung/rente/altersrente/altersrente.py	85.71%	1 Missing ⚠️

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5 files changed · ± 5 modified

± Modified

• changes.html
• geps/gep-03.html
• how_to_guides/modifications_of_policy_environments.html
• tutorials/simple_example.html
• tutorials/visualizing_the_system.html