[henrydingliu]

Summary of Changes

More clean-up and bug fix as I work through Chapter 16

• Cleaning up zeta_-related properties into IncrementalMixin, matching DisposalMixin
• fixed a bug in cum_to_incr for cum_zeta_, w/ new test to guard future drift; also improved readability of the surrounding code
• refactored _param_property across modules to condense the universal properties to DevelopmentBase and keep module-specific properties at module level
• other docstring improvements

Related GitHub Issue(s)

#588
#377

AI/LLM Usage

AI did some autofill of docstrings, which I then validated edited

Additional Context for Reviewers

zeta_ still shows the wrong development labels. will address after #1304

Submitter's Checklist

• I have reviewed and am adhering to the standards outlined in the project Governing Doc.
• The PR subject title summarizes the changes, with one proper prefix ([FIX], [FEAT], [DOCS], [TST], [CHORE], or [BRK]).
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Reviewer's Checklist

• The implementation addresses the associated issue(s).
• The implementation is appropriate, maintainable, and follows ARCHITECTURE.md.
• PR subject title has the proper prefix and the subject is appropriate.
• Relevant issue(s) are linked.
• AI/LLM usage is disclosed and appropriate.
• Documentation and tests are appropriate.
• CI tests passed, or any failures are acceptable.
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---

Note

Medium Risk
Changes triangle incr/cum conversion and how fitted development parameters are tagged, which can affect incremental additive and pattern-based workflows; mitigated by targeted tests.

Overview
Incremental additive (zeta_) handling is aligned with the disposal mixin pattern: a new IncrementalMixin on TriangleBase exposes zeta_ / cum_zeta_ with setters that stamp pattern flags (is_pattern, is_additive, is_cumulative), stores values on _zeta_, and has_zeta now keys off that private attribute. cum_zeta_ is removed from the shared Common mixin in favor of the incremental mixin API.

Parameter triangles are refactored so DevelopmentBase._param_property only sets shared pattern metadata; Development, IncrementalAdditive, and disposal estimators override it for type-specific flags (multiplicative LDF vs additive zeta_ vs disposal rates). Development.fit no longer stacks regression outputs into one array with an index—each statistic gets its own _param_property call.

incr_to_cum / cum_to_incr on Triangle are rewritten with explicit branches for multiplicative LDF patterns, additive factor patterns (is_additive), normal increments, and disposal rates, plus a dedicated is_additive property. This fixes incorrect incremental conversion for cumulative additive (cum_zeta_) patterns.

Tests cover setting cum_zeta_, missing zeta_ errors, and updated has_zeta / _param_property call sites.

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[github-actions]

Pyright Type Completeness

View the full pyright --verifytypes output for this commit

Project (full chainladder package, at this PR's head): 15.1% of exported symbols fully typed (209 / 1387)

	Known	Ambiguous	Unknown	Total
Project (head)	209	111	1067	1387

Other symbols referenced but not exported by chainladder: 13

	Known	Ambiguous	Unknown	Total
Other (head)	3	1	9	13

Symbols without documentation:

• Functions without docstring: 327
• Functions without default param: 0
• Classes without docstring: 10

Patch (exported symbols added or changed by this PR): 33.3% fully typed (2 / 6); 1 no longer exported

	Known	Ambiguous	Unknown	Total
Patch	2	0	4	6

Patch symbol details

Symbol	Status	Change
chainladder.core.triangle.Triangle.is_additive	✅ known	new
chainladder.development.incremental.IncrementalMixin	❌ unknown	new
chainladder.development.incremental.IncrementalMixin.cum_zeta_	❌ unknown	new
chainladder.development.incremental.IncrementalMixin.zeta_	❌ unknown	new
chainladder.development.tests.test_incremental.test_no_zeta	❌ unknown	new
chainladder.development.tests.test_incremental.test_setting_cum	✅ known	new
chainladder.core.common.Common.cum_zeta_	—	no longer exported (was ❌ unknown)

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.95%. Comparing base (d82c521) to head (4b3e652).
⚠️ Report is 119 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1419      +/-   ##
==========================================
+ Coverage   91.79%   92.95%   +1.16%     
==========================================
  Files          96      102       +6     
  Lines        5494     6092     +598     
  Branches      707      771      +64     
==========================================
+ Hits         5043     5663     +620     
+ Misses        327      310      -17     
+ Partials      124      119       -5     

Flag	Coverage Δ
unittests	92.95% <100.00%> (+1.16%)	⬆️

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[henrydingliu]

@genedan just assigned this to you as i'd like this to land so i can continue working on chapter 16.

[genedan]

I'm working my way through the open PRs bottom up, so this one is next.

[henrydingliu]

I'm working my way through the open PRs bottom up, so this one is next.

thanks. the series of #840 PRs are lower prio than #1455 and #1480

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