Parameterize 25% on line 37 of AMT

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: patch
+  changes:
+    fixes:
+    - Parameterized 25% of the Alternative Minimum Tax calculation.

policyengine_us/parameters/gov/irs/income/amt/capital_gains/capital_gain_excess_tax_rate.yaml

@@ -0,0 +1,10 @@
+description: The IRS multiplies the taxable excess capital gain by this rate.
+metadata:
+  unit: /1
+  label: Alternative Minimum Tax capital gain excess tax rate
+  period: year
+  reference:
+    - title: 26 U.S. Code § 55 - Alternative minimum tax imposed
+      href: https://www.law.cornell.edu/uscode/text/26/55#b_3_E
+values:
+  2021-01-01: 0.25

policyengine_us/variables/gov/irs/tax/federal_income/alternative_minimum_tax.py

@@ -75,15 +75,18 @@ def formula(tax_unit, period, parameters):
         dwks34 = dwks10 + dwks19
         dwks36 = max_(0, dwks34 - dwks1)
         dwks37 = max_(0, dwks33 - dwks36)
-        dwks38 = 0.25 * dwks37
+
+        # parameterized
+        p = parameters(period).gov.irs.income
+
+        dwks38 = p.amt.capital_gain_excess_tax_rate * dwks37
         # Break in worksheet lines
         dwks39 = dwks19 + dwks20 + dwks28 + dwks31 + dwks37
         dwks40 = dwks1 - dwks39
         dwks41 = 0.28 * dwks40
 
         # Compute regular tax using bracket rates and thresholds
         reg_taxinc = max_(0, dwks19)
-        p = parameters(period).gov.irs.income
         bracket_tops = p.bracket.thresholds
         bracket_rates = p.bracket.rates
         reg_tax = 0