Implement Maine PTFC unknown-utilities 15% rule (Schedule PTFC line 5c)

[DTrim99]

Surfaced by PolicyEngine/policyengine-taxsim#1126 (ME renter with rent that includes utilities).

Problem

Maine's property tax fairness credit treats 15% of rent as "rent constituting property taxes," after first excluding any heat/utilities included in the rent. Per 2022 Schedule PTFC/STFC line 5c: if the rent includes utilities and the amount is known, subtract it; if the rent includes utilities and the amount is not known, subtract 15% of gross rent instead.

me_property_tax_fairness_credit_countable_rent only handled the known-amount case (utilities_included_in_rent * utility_expense) and its comment noted the unknown-amount branch was "not implemented." So a filer whose rent includes utilities but who did not itemize a utility amount received no utility exclusion at all, overstating the countable rent (and the credit).

Fix

• Implement the line-5c fallback: when utilities_included_in_rent is true and the utility amount is unknown (utility_expense is 0), deduct 15% of gross rent before applying the 15% rent-constituting-property-tax rate.
• Add parameter gov.states.me.tax.income.credits.fairness.property_tax.rate.utilities_included_in_rent (0.15) for the utility fraction.
• Add a countable-rent test for the unknown-amount case.

Example

Rent $17,139 with utilities included, amount unknown: utility portion = 15% × 17,139 = 2,571; countable rent = 15% × (17,139 − 2,571) = 2,185, matching the Maine worksheet (previously PE used the full 15% × 17,139 = 2,571). The known-amount and utilities-not-included cases are unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a6acf1f) to head (bea2956).
⚠️ Report is 204 commits behind head on main.

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[PavelMakarchuk]

The implementation mechanics check out — all 34 tests in the ME PTFC suite pass on the branch, the new parameter loads with correct metadata, utilities_included_in_rent defaults to false so microsim defaults are unaffected, no partner contract tests are touched, and the worksheet math in the new test (127.5 = (1000 − 150) × 0.15) is internally consistent with the quoted form text.

Requesting changes for two findings on the rewritten formula, detailed inline:

1. utility_expense is the wrong "amount known" discriminator — it's the household's general SNAP-style utility expense total, not the utility portion of rent, so the new 15% fallback only fires when the household reports zero utility expenses of any kind, and separately-paid utilities get subtracted from rent.
2. No clamp when utilities exceed rent — countable rent can go negative and offsets real_estate_taxes downstream; line 5d on the form can never be negative.

Caveat: maine.gov was unreachable from the review environment, so the line-5c rule was verified against the PR's quoted form text and worksheet example rather than the form PDF itself.

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[DTrim99]

Fixes applied from @PavelMakarchuk's review

Both findings addressed and pushed (ME PTFC suite 36/36). Thanks for the catch — both were masked by the old tests.

F1 — wrong known/unknown discriminator (fixed)

You're right that utility_expense (the SPM-unit's general SNAP-style utility total) is the wrong signal for "is the utility portion of rent known?" — it made the line-5c 15% fallback unreachable for any household reporting utility expenses and subtracted separately-paid utilities from rent. Added a dedicated me_ptfc_utilities_included_in_rent (TaxUnit, USD, default 0) and rewired the split off the utilities_included_in_rent boolean + that amount:

• not included in rent → no subtraction (full rent counts);
• included & amount known (> 0) → subtract that amount;
• included & amount unknown (0) → subtract the 15%-of-rent estimate (line 5c).

Your concrete case now resolves to the form's (12,000 − 1,800) × 0.15 = 1,530 instead of 1,710, and the separately-paid utility_expense is no longer subtracted.

F2 — clamp when utilities exceed rent (fixed)

net_rent is now max_(rent − utility_portion, 0), so countable rent can never go negative and offset real_estate_taxes downstream (line 5d ≥ 0). Your rent = 2,400 / utilities = 3,600 → −180 case now returns 0.

Tests

Appended two demonstrating cases that fail if either fix is reverted:

• separately-paid-utilities (rent 12,000, included-amount unknown, general utility_expense 600) → 1,530 (would be 1,710 under the old discriminator);
• utilities-exceed-rent (rent 2,400, included-amount 3,600) → 0 (would be −180 unclamped).

The existing known-amount case (1000 − 150) × 0.15 = 127.5 still passes. No parameter values changed (the 15% rates were already added by this PR).

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