From 8b83973987d1a3fbe541018ff037df290ed741c1 Mon Sep 17 00:00:00 2001
From: "henrydingliu@gmail.com" <henrydingliu@gmail.com>
Date: Thu, 17 Sep 2026 07:14:05 +0000
Subject: [PATCH 01/11] overnight dump

---
 chainladder/adjustments/disposal.py           |  7 +++
 chainladder/core/base.py                      |  2 +
 chainladder/core/common.py                    |  9 +--
 chainladder/core/triangle.py                  | 13 ++++
 chainladder/development/base.py               |  3 -
 chainladder/development/development.py        |  8 +--
 chainladder/development/incremental.py        | 61 +++++++++++++++++++
 .../development/tests/test_incremental.py     | 20 ++++++
 8 files changed, 105 insertions(+), 18 deletions(-)

diff --git a/chainladder/adjustments/disposal.py b/chainladder/adjustments/disposal.py
index fcd3a568f..79f9ccb46 100644
--- a/chainladder/adjustments/disposal.py
+++ b/chainladder/adjustments/disposal.py
@@ -339,3 +339,10 @@ def fit_transform(self, X, y=None, sample_weight=None):
             Triangle with new full_triangle_
         """
         return self.fit(X, y, sample_weight).transform(X, sample_weight=sample_weight)
+
+    def _param_property(self, X, params, idx):
+        obj = super._param_property(X, params)
+        obj.values = params
+        obj.is_disposal_rate = True
+
+        return obj
\ No newline at end of file
diff --git a/chainladder/core/base.py b/chainladder/core/base.py
index ac89fc131..ae20ae43b 100644
--- a/chainladder/core/base.py
+++ b/chainladder/core/base.py
@@ -28,6 +28,7 @@
 from chainladder.utils.sparse import sp
 
 from chainladder.adjustments.disposal import DisposalMixin
+from chainladder.development.incremental import IncrementalMixin
 
 from typing import Optional, TYPE_CHECKING
 
@@ -50,6 +51,7 @@ class TriangleBase(
     Common,
     ABC,
     DisposalMixin,
+    IncrementalMixin,
 ):
     """This class handles the initialization of a triangle"""
 
diff --git a/chainladder/core/common.py b/chainladder/core/common.py
index 8f421cb51..53eee285d 100644
--- a/chainladder/core/common.py
+++ b/chainladder/core/common.py
@@ -82,7 +82,7 @@ def has_ldf(self):
 
     @property
     def has_zeta(self):
-        if hasattr(self, "zeta_"):
+        if hasattr(self, "_zeta_"):
             return True
         else:
             return False
@@ -159,13 +159,6 @@ def pct_unreported_(self):
             )
         return 1 - 1 / self.cdf_
 
-    @property
-    def cum_zeta_(self):
-        if not self.has_zeta:
-            x = self.__class__.__name__
-            raise AttributeError("'" + x + "' object has no attribute 'cum_zeta_'")
-        return self.zeta_.incr_to_cum()
-
     @property
     def ibnr_(self):
         """
diff --git a/chainladder/core/triangle.py b/chainladder/core/triangle.py
index a496d7edc..adfd54e58 100644
--- a/chainladder/core/triangle.py
+++ b/chainladder/core/triangle.py
@@ -1046,6 +1046,19 @@ def is_disposal_rate(self) -> bool:
     def is_disposal_rate(self, is_dr: bool) -> None:
         self._is_disposal_rate = is_dr
 
+    @property
+    def is_additive(self) -> bool:
+        """
+        Indicates whether the Triangle holds a normal additive ratio (which goes from tail to head)
+        """
+        if hasattr(self, "_is_additive"):
+            return self._is_additive
+        return False
+
+    @is_additive.setter
+    def is_additive(self, is_add: bool) -> None:
+        self._is_additive = is_add
+
     def align_pattern(
         self, X: Triangle, sample_weight: Triangle | None = None
     ) -> Triangle:
diff --git a/chainladder/development/base.py b/chainladder/development/base.py
index 922c4dbc2..4c09067da 100644
--- a/chainladder/development/base.py
+++ b/chainladder/development/base.py
@@ -409,11 +409,8 @@ def _param_property(X: Triangle, params: np.ndarray) -> Triangle:
         from chainladder import options
 
         obj: Triangle = X[X.origin == X.origin.min()]
-        obj.values = params
         obj.valuation_date = pd.to_datetime(options.ULT_VAL)
         obj.is_pattern = True
-        obj.is_additive = True
-        obj.is_cumulative = False
         obj.virtual_columns.columns = {}
         obj._set_slicers()
         return obj
diff --git a/chainladder/development/development.py b/chainladder/development/development.py
index 705e955fe..451a50233 100644
--- a/chainladder/development/development.py
+++ b/chainladder/development/development.py
@@ -482,16 +482,10 @@ def transform(self, X):
         return X_new
 
     def _param_property(self, X, params, idx):
-        from chainladder import options
-
-        obj = X[X.origin == X.origin.min()]
+        obj = super._param_property(X, params)
         xp = X.get_array_module()
         obj.values = xp.ones(obj.shape)[..., :-1] * params[..., idx : idx + 1, :]
         obj.ddims = X.link_ratio.ddims
-        obj.valuation_date = pd.to_datetime(options.ULT_VAL)
-        obj.is_pattern = True
         obj.is_cumulative = False
-        obj.virtual_columns.columns = {}
-        obj._set_slicers()
 
         return obj
diff --git a/chainladder/development/incremental.py b/chainladder/development/incremental.py
index 0d7195af3..407074402 100644
--- a/chainladder/development/incremental.py
+++ b/chainladder/development/incremental.py
@@ -1,11 +1,65 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at https://mozilla.org/MPL/2.0/.
+from __future__ import annotations
+
 from chainladder.development import DevelopmentBase
 from chainladder.utils import TriangleWeight
 import numpy as np
+import copy
 import warnings
 
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from chainladder.core import Triangle
+
+
+class IncrementalMixin:
+    """
+    This class provides attributes for the IncrementalAdditive development method
+    and transformed `Triangle`
+    """
+
+    @property
+    def zeta_(self) -> Triangle:
+        """
+        Gets the estimated zeta
+        """
+        if not hasattr(self, "_zeta_"):
+            x = self.__class__.__name__
+            raise AttributeError("'" + x + "' object has no attribute 'zeta_'")
+        return self._zeta_
+
+    @zeta_.setter
+    def zeta_(self, value: Triangle) -> None:
+        """
+        Sets zeta_
+        """
+        obj = copy.deepcopy(value)
+        obj.is_pattern = True
+        obj.is_additive = True
+        obj.is_cumulative = False
+        self._zeta_ = obj
+
+    @property
+    def cum_zeta_(self) -> Triangle:
+        """
+        Gets the cumulative of the estimated zeta
+        """
+        return self.zeta_.incr_to_cum()
+
+    @cum_zeta_.setter
+    def cum_zeta_(self, value: Triangle) -> None:
+        """
+        Sets cum_zeta_
+        """
+        obj = copy.deepcopy(value)
+        obj.is_pattern = True
+        obj.is_additive = True
+        obj.is_cumulative = True
+        self._zeta_ = obj.cum_to_incr()
+
 
 class IncrementalAdditive(DevelopmentBase):
     """
@@ -319,3 +373,10 @@ def transform(self, X):
         ]:
             X_new.__dict__[item] = self.__dict__[item]
         return X_new
+
+    def _param_property(self, X, params, idx):
+        obj = super._param_property(X, params)
+        obj.values = params
+        obj.is_additive = True
+
+        return obj
\ No newline at end of file
diff --git a/chainladder/development/tests/test_incremental.py b/chainladder/development/tests/test_incremental.py
index 58c6ed3c1..597694b24 100644
--- a/chainladder/development/tests/test_incremental.py
+++ b/chainladder/development/tests/test_incremental.py
@@ -1,7 +1,14 @@
+from __future__ import annotations
+
 import chainladder as cl
 import numpy as np
 import pytest
 
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from chainladder.core import Triangle
+
 
 def test_schmidt():
     tri = cl.load_sample("ia_sample")
@@ -90,3 +97,16 @@ def test_pipeline():
     assert np.array_equal(
         dev1.zeta_.values, dev2.named_steps.drop_hilo.zeta_.values, True
     )
+
+
+def test_setting_cum() -> None:
+    """
+    IncrementalMixin allows setting cumulative zeta. Validating that the setting function works properly.
+    """
+    tri = cl.load_sample("ia_sample")
+    ia = cl.IncrementalAdditive().fit(
+        tri.iloc[0, 0], sample_weight=tri.iloc[0, 1].latest_diagonal
+    )
+    orig_zeta_ = ia.zeta_
+    ia.cum_zeta_ = ia.cum_zeta_ * 2
+    assert ia.zeta_ == orig_zeta_ * 2

From 988e42dfd519e6ae544533757473d9bb7fa289ff Mon Sep 17 00:00:00 2001
From: "henrydingliu@gmail.com" <henrydingliu@gmail.com>
Date: Thu, 17 Sep 2026 08:22:11 +0000
Subject: [PATCH 02/11] bug fix

---
 chainladder/adjustments/disposal.py    | 2 +-
 chainladder/development/development.py | 2 +-
 chainladder/development/incremental.py | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/chainladder/adjustments/disposal.py b/chainladder/adjustments/disposal.py
index 79f9ccb46..fbbbe401a 100644
--- a/chainladder/adjustments/disposal.py
+++ b/chainladder/adjustments/disposal.py
@@ -341,7 +341,7 @@ def fit_transform(self, X, y=None, sample_weight=None):
         return self.fit(X, y, sample_weight).transform(X, sample_weight=sample_weight)
 
     def _param_property(self, X, params, idx):
-        obj = super._param_property(X, params)
+        obj = super()._param_property(X, params)
         obj.values = params
         obj.is_disposal_rate = True
 
diff --git a/chainladder/development/development.py b/chainladder/development/development.py
index 451a50233..4a2734d66 100644
--- a/chainladder/development/development.py
+++ b/chainladder/development/development.py
@@ -482,7 +482,7 @@ def transform(self, X):
         return X_new
 
     def _param_property(self, X, params, idx):
-        obj = super._param_property(X, params)
+        obj = super()._param_property(X, params)
         xp = X.get_array_module()
         obj.values = xp.ones(obj.shape)[..., :-1] * params[..., idx : idx + 1, :]
         obj.ddims = X.link_ratio.ddims
diff --git a/chainladder/development/incremental.py b/chainladder/development/incremental.py
index 407074402..a19248f95 100644
--- a/chainladder/development/incremental.py
+++ b/chainladder/development/incremental.py
@@ -375,7 +375,7 @@ def transform(self, X):
         return X_new
 
     def _param_property(self, X, params, idx):
-        obj = super._param_property(X, params)
+        obj = super()._param_property(X, params)
         obj.values = params
         obj.is_additive = True
 

From af3c621c56743febccdca1c510ed12d3ec9f569a Mon Sep 17 00:00:00 2001
From: "henrydingliu@gmail.com" <henrydingliu@gmail.com>
Date: Thu, 17 Sep 2026 08:31:16 +0000
Subject: [PATCH 03/11] various updates

---
 chainladder/adjustments/disposal.py               | 4 ++--
 chainladder/development/development.py            | 1 -
 chainladder/development/incremental.py            | 4 ++--
 chainladder/development/tests/test_incremental.py | 7 -------
 4 files changed, 4 insertions(+), 12 deletions(-)

diff --git a/chainladder/adjustments/disposal.py b/chainladder/adjustments/disposal.py
index fbbbe401a..7b13c329a 100644
--- a/chainladder/adjustments/disposal.py
+++ b/chainladder/adjustments/disposal.py
@@ -340,9 +340,9 @@ def fit_transform(self, X, y=None, sample_weight=None):
         """
         return self.fit(X, y, sample_weight).transform(X, sample_weight=sample_weight)
 
-    def _param_property(self, X, params, idx):
+    def _param_property(self, X, params):
         obj = super()._param_property(X, params)
         obj.values = params
         obj.is_disposal_rate = True
 
-        return obj
\ No newline at end of file
+        return obj
diff --git a/chainladder/development/development.py b/chainladder/development/development.py
index 4a2734d66..e9b0f8f04 100644
--- a/chainladder/development/development.py
+++ b/chainladder/development/development.py
@@ -4,7 +4,6 @@
 from __future__ import annotations
 
 import numpy as np
-import pandas as pd
 import warnings
 
 from chainladder.development.base import DevelopmentBase
diff --git a/chainladder/development/incremental.py b/chainladder/development/incremental.py
index a19248f95..0c83e7060 100644
--- a/chainladder/development/incremental.py
+++ b/chainladder/development/incremental.py
@@ -374,9 +374,9 @@ def transform(self, X):
             X_new.__dict__[item] = self.__dict__[item]
         return X_new
 
-    def _param_property(self, X, params, idx):
+    def _param_property(self, X, params):
         obj = super()._param_property(X, params)
         obj.values = params
         obj.is_additive = True
 
-        return obj
\ No newline at end of file
+        return obj
diff --git a/chainladder/development/tests/test_incremental.py b/chainladder/development/tests/test_incremental.py
index 597694b24..751c9b3d0 100644
--- a/chainladder/development/tests/test_incremental.py
+++ b/chainladder/development/tests/test_incremental.py
@@ -1,14 +1,7 @@
-from __future__ import annotations
-
 import chainladder as cl
 import numpy as np
 import pytest
 
-from typing import TYPE_CHECKING
-
-if TYPE_CHECKING:
-    from chainladder.core import Triangle
-
 
 def test_schmidt():
     tri = cl.load_sample("ia_sample")

From aa083d86fcb3b8e23ec3b88839ab25d3479cd5b0 Mon Sep 17 00:00:00 2001
From: "henrydingliu@gmail.com" <henrydingliu@gmail.com>
Date: Thu, 17 Sep 2026 15:30:36 +0000
Subject: [PATCH 04/11] fixing tests

---
 chainladder/core/tests/test_triangle.py       |  4 +-
 chainladder/core/triangle.py                  | 61 ++++++++++++++-----
 chainladder/development/incremental.py        |  4 +-
 .../development/tests/test_incremental.py     | 15 ++++-
 4 files changed, 62 insertions(+), 22 deletions(-)

diff --git a/chainladder/core/tests/test_triangle.py b/chainladder/core/tests/test_triangle.py
index 45c1e649b..c89ecb00d 100644
--- a/chainladder/core/tests/test_triangle.py
+++ b/chainladder/core/tests/test_triangle.py
@@ -3200,7 +3200,7 @@ def test_has_zeta_true(raa: Triangle) -> None:
     None
     """
     fitted = cl.IncrementalAdditive().fit(raa, sample_weight=raa.latest_diagonal)
-    assert fitted.has_zeta is True
+    assert fitted.has_zeta
 
 
 def test_has_zeta_false(raa: Triangle) -> None:
@@ -3216,7 +3216,7 @@ def test_has_zeta_false(raa: Triangle) -> None:
     -------
     None
     """
-    assert cl.Development().fit(raa).has_zeta is False
+    assert not cl.Development().fit(raa).has_zeta
 
 
 def test_cum_zeta_raises_when_no_zeta(raa: Triangle) -> None:
diff --git a/chainladder/core/triangle.py b/chainladder/core/triangle.py
index adfd54e58..c85333ecf 100644
--- a/chainladder/core/triangle.py
+++ b/chainladder/core/triangle.py
@@ -1390,20 +1390,28 @@ def incr_to_cum(self, inplace=False):
         if inplace:
             xp = self.get_array_module()
             if not self.is_cumulative:
-                if self.is_pattern & (not self.is_disposal_rate):
-                    if hasattr(self, "is_additive") and self.is_additive:
-                        values = xp.nan_to_num(self.values[..., ::-1])
-                        values = num_to_value(values, 0)
-                        self.values = (
-                            xp.cumsum(values, -1)[..., ::-1] * self.nan_triangle
-                        )
-                    else:
-                        values = xp.nan_to_num(self.values[..., ::-1])
-                        values = num_to_value(values, 1)
-                        values = xp.cumprod(values, -1)[..., ::-1]
-                        self.values = values * self.nan_triangle
-                        values = num_to_value(values, self.get_array_module(values).nan)
-                else:
+                if (
+                    self.is_pattern
+                    and (not self.is_additive)
+                    and (not self.is_disposal_rate)
+                ):  # for multiplicative ldf triangles (mult from tail to head)
+                    values = xp.nan_to_num(self.values[..., ::-1])
+                    values = num_to_value(values, 1)
+                    values = xp.cumprod(values, -1)[..., ::-1]
+                    self.values = values * self.nan_triangle
+                    values = num_to_value(values, self.get_array_module(values).nan)
+                elif (
+                    self.is_pattern
+                    and self.is_additive
+                    and (not self.is_disposal_rate)
+                ):  # for additive factor triangles (sum from tail to head)
+                    values = xp.nan_to_num(self.values[..., ::-1])
+                    values = num_to_value(values, 0)
+                    self.values = (
+                        xp.cumsum(values, -1)[..., ::-1] * self.nan_triangle
+                    )
+                else:  # for normal value triangles 
+                # also for disposal rate triangles (sum from head to tail)
                     if self.array_backend not in ["sparse", "dask"]:
                         self.values = (
                             xp.cumsum(xp.nan_to_num(self.values), 3)
@@ -1479,7 +1487,11 @@ def cum_to_incr(self, inplace=False):
         if inplace:
             v = self.valuation_date
             if self.is_cumulative or self.is_cumulative is None:
-                if self.is_pattern & (not self.is_disposal_rate):
+                if (
+                    self.is_pattern
+                    and (not self.is_additive)
+                    and (not self.is_disposal_rate)
+                ):  # for multiplicative ldf triangles (div from head to tail)
                     xp = self.get_array_module()
                     self.values = num_to_value(xp.nan_to_num(self.values), 1)
                     diff = self.iloc[..., :-1] / self.iloc[..., 1:].values
@@ -1491,7 +1503,24 @@ def cum_to_incr(self, inplace=False):
                         axis=3,
                     )
                     self.values = self.values * self.nan_triangle
-                else:
+                elif (
+                    self.is_pattern
+                    and self.is_additive
+                    and (not self.is_disposal_rate)
+                ):  # for additive factor triangles (diff from head to tail)
+                    xp = self.get_array_module()
+                    self.values = num_to_value(xp.nan_to_num(self.values), 1)
+                    diff = self.iloc[..., :-1] - self.iloc[..., 1:].values
+                    self = concat(
+                        (
+                            diff,
+                            self.iloc[..., -1],
+                        ),
+                        axis=3,
+                    )
+                    self.values = self.values * self.nan_triangle
+                else:  # for normal value triangles 
+                # also for disposal rate triangles (sum from head to tail)
                     diff = self.iloc[..., 1:] - self.iloc[..., :-1].values
                     self = concat((self.iloc[..., 0], diff), axis=3)
                 self.is_cumulative = False
diff --git a/chainladder/development/incremental.py b/chainladder/development/incremental.py
index 0c83e7060..b7af2e644 100644
--- a/chainladder/development/incremental.py
+++ b/chainladder/development/incremental.py
@@ -61,7 +61,7 @@ def cum_zeta_(self, value: Triangle) -> None:
         self._zeta_ = obj.cum_to_incr()
 
 
-class IncrementalAdditive(DevelopmentBase):
+class IncrementalAdditive(DevelopmentBase, IncrementalMixin):
     """
     The Incremental Additive Method.
 
@@ -365,7 +365,7 @@ def transform(self, X):
         for item in [
             "ldf_",
             "w_",
-            "zeta_",
+            "_zeta_",
             "incremental_",
             "tri_zeta",
             "fit_zeta_",
diff --git a/chainladder/development/tests/test_incremental.py b/chainladder/development/tests/test_incremental.py
index 751c9b3d0..8c5ef4d2a 100644
--- a/chainladder/development/tests/test_incremental.py
+++ b/chainladder/development/tests/test_incremental.py
@@ -1,7 +1,14 @@
+from __future__ import annotations
+
 import chainladder as cl
 import numpy as np
 import pytest
 
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from chainladder import Triangle
+
 
 def test_schmidt():
     tri = cl.load_sample("ia_sample")
@@ -92,7 +99,7 @@ def test_pipeline():
     )
 
 
-def test_setting_cum() -> None:
+def test_setting_cum(atol: float) -> None:
     """
     IncrementalMixin allows setting cumulative zeta. Validating that the setting function works properly.
     """
@@ -102,4 +109,8 @@ def test_setting_cum() -> None:
     )
     orig_zeta_ = ia.zeta_
     ia.cum_zeta_ = ia.cum_zeta_ * 2
-    assert ia.zeta_ == orig_zeta_ * 2
+    assert np.allclose(
+        ia.zeta_.values,
+        (orig_zeta_ * 2).values,
+        atol=atol
+    )

From da1257e21ba9ef12281fac041810ae341b68fd97 Mon Sep 17 00:00:00 2001
From: "henrydingliu@gmail.com" <henrydingliu@gmail.com>
Date: Mon, 21 Sep 2026 16:50:47 +0000
Subject: [PATCH 05/11] improving docstring and bugbot fix

---
 chainladder/adjustments/disposal.py    | 20 +++++++++++++++++++-
 chainladder/core/triangle.py           | 25 ++++++++-----------------
 chainladder/development/development.py | 18 ++++++++++++++++++
 chainladder/development/incremental.py | 22 ++++++++++++++++++++--
 4 files changed, 65 insertions(+), 20 deletions(-)

diff --git a/chainladder/adjustments/disposal.py b/chainladder/adjustments/disposal.py
index 7b13c329a..2e4e3fa06 100644
--- a/chainladder/adjustments/disposal.py
+++ b/chainladder/adjustments/disposal.py
@@ -340,7 +340,25 @@ def fit_transform(self, X, y=None, sample_weight=None):
         """
         return self.fit(X, y, sample_weight).transform(X, sample_weight=sample_weight)
 
-    def _param_property(self, X, params):
+    def _param_property(self, X: Triangle, params: np.ndarray) -> Triangle:
+        """
+        Private method to wrap a numpy array into a Triangle
+
+        Uses and overrides DevelopmentBase._param_property.
+
+        Parameters
+        ----------
+        X : Triangle
+            The Triangle object that we want to place the parameter into
+
+        params : np.ndarray
+            The parammeter we want to turn into a Triangle
+
+        Returns
+        -------
+        Triangle
+            A Triangle that mimics X but has params values
+        """
         obj = super()._param_property(X, params)
         obj.values = params
         obj.is_disposal_rate = True
diff --git a/chainladder/core/triangle.py b/chainladder/core/triangle.py
index c85333ecf..fb44b75b1 100644
--- a/chainladder/core/triangle.py
+++ b/chainladder/core/triangle.py
@@ -1394,7 +1394,7 @@ def incr_to_cum(self, inplace=False):
                     self.is_pattern
                     and (not self.is_additive)
                     and (not self.is_disposal_rate)
-                ):  # for multiplicative ldf triangles (mult from tail to head)
+                ):  # for multiplicative ldf triangles (mult from left to right)
                     values = xp.nan_to_num(self.values[..., ::-1])
                     values = num_to_value(values, 1)
                     values = xp.cumprod(values, -1)[..., ::-1]
@@ -1404,14 +1404,14 @@ def incr_to_cum(self, inplace=False):
                     self.is_pattern
                     and self.is_additive
                     and (not self.is_disposal_rate)
-                ):  # for additive factor triangles (sum from tail to head)
+                ):  # for additive factor triangles (sum from left to right)
                     values = xp.nan_to_num(self.values[..., ::-1])
                     values = num_to_value(values, 0)
                     self.values = (
                         xp.cumsum(values, -1)[..., ::-1] * self.nan_triangle
                     )
                 else:  # for normal value triangles 
-                # also for disposal rate triangles (sum from head to tail)
+                # also for disposal rate triangles (sum from right to left)
                     if self.array_backend not in ["sparse", "dask"]:
                         self.values = (
                             xp.cumsum(xp.nan_to_num(self.values), 3)
@@ -1491,7 +1491,7 @@ def cum_to_incr(self, inplace=False):
                     self.is_pattern
                     and (not self.is_additive)
                     and (not self.is_disposal_rate)
-                ):  # for multiplicative ldf triangles (div from head to tail)
+                ):  # for multiplicative ldf triangles (div from left to right)
                     xp = self.get_array_module()
                     self.values = num_to_value(xp.nan_to_num(self.values), 1)
                     diff = self.iloc[..., :-1] / self.iloc[..., 1:].values
@@ -1507,20 +1507,11 @@ def cum_to_incr(self, inplace=False):
                     self.is_pattern
                     and self.is_additive
                     and (not self.is_disposal_rate)
-                ):  # for additive factor triangles (diff from head to tail)
-                    xp = self.get_array_module()
-                    self.values = num_to_value(xp.nan_to_num(self.values), 1)
-                    diff = self.iloc[..., :-1] - self.iloc[..., 1:].values
-                    self = concat(
-                        (
-                            diff,
-                            self.iloc[..., -1],
-                        ),
-                        axis=3,
-                    )
-                    self.values = self.values * self.nan_triangle
+                ):  # for additive factor triangles (diff from left to right)
+                    diff = self.iloc[..., :-1].values - self.iloc[..., 1:]
+                    self = concat((diff, self.iloc[..., -1]), axis=3)
                 else:  # for normal value triangles 
-                # also for disposal rate triangles (sum from head to tail)
+                # also for disposal rate triangles (diff from right to left)
                     diff = self.iloc[..., 1:] - self.iloc[..., :-1].values
                     self = concat((self.iloc[..., 0], diff), axis=3)
                 self.is_cumulative = False
diff --git a/chainladder/development/development.py b/chainladder/development/development.py
index e9b0f8f04..3559d65cd 100644
--- a/chainladder/development/development.py
+++ b/chainladder/development/development.py
@@ -481,6 +481,24 @@ def transform(self, X):
         return X_new
 
     def _param_property(self, X, params, idx):
+        """
+        Private method to wrap a numpy array into a Triangle
+
+        Uses and overrides DevelopmentBase._param_property.
+
+        Parameters
+        ----------
+        X : Triangle
+            The Triangle object that we want to place the parameter into
+
+        params : np.ndarray
+            The parammeter we want to turn into a Triangle
+
+        Returns
+        -------
+        Triangle
+            A Triangle that mimics X but has params values
+        """
         obj = super()._param_property(X, params)
         xp = X.get_array_module()
         obj.values = xp.ones(obj.shape)[..., :-1] * params[..., idx : idx + 1, :]
diff --git a/chainladder/development/incremental.py b/chainladder/development/incremental.py
index b7af2e644..f809bc30b 100644
--- a/chainladder/development/incremental.py
+++ b/chainladder/development/incremental.py
@@ -24,7 +24,7 @@ class IncrementalMixin:
     @property
     def zeta_(self) -> Triangle:
         """
-        Gets the estimated zeta
+        Gets the estimated incremental additive factors
         """
         if not hasattr(self, "_zeta_"):
             x = self.__class__.__name__
@@ -45,7 +45,7 @@ def zeta_(self, value: Triangle) -> None:
     @property
     def cum_zeta_(self) -> Triangle:
         """
-        Gets the cumulative of the estimated zeta
+        Gets the estimated cumulative additive factors
         """
         return self.zeta_.incr_to_cum()
 
@@ -375,6 +375,24 @@ def transform(self, X):
         return X_new
 
     def _param_property(self, X, params):
+        """
+        Private method to wrap a numpy array into a Triangle
+
+        Uses and overrides DevelopmentBase._param_property.
+
+        Parameters
+        ----------
+        X : Triangle
+            The Triangle object that we want to place the parameter into
+
+        params : np.ndarray
+            The parammeter we want to turn into a Triangle
+
+        Returns
+        -------
+        Triangle
+            A Triangle that mimics X but has params values
+        """
         obj = super()._param_property(X, params)
         obj.values = params
         obj.is_additive = True

From 87228e613a23630a3b077e966a70f74f5818b8a3 Mon Sep 17 00:00:00 2001
From: "henrydingliu@gmail.com" <henrydingliu@gmail.com>
Date: Mon, 21 Sep 2026 17:18:51 +0000
Subject: [PATCH 06/11] unittest & ruff fix

---
 chainladder/core/triangle.py                    | 17 ++++++++++++-----
 .../development/tests/test_incremental.py       | 11 +----------
 2 files changed, 13 insertions(+), 15 deletions(-)

diff --git a/chainladder/core/triangle.py b/chainladder/core/triangle.py
index fb44b75b1..891eb6367 100644
--- a/chainladder/core/triangle.py
+++ b/chainladder/core/triangle.py
@@ -1390,28 +1390,30 @@ def incr_to_cum(self, inplace=False):
         if inplace:
             xp = self.get_array_module()
             if not self.is_cumulative:
+                # fmt: off
                 if (
                     self.is_pattern
                     and (not self.is_additive)
                     and (not self.is_disposal_rate)
                 ):  # for multiplicative ldf triangles (mult from left to right)
+                    # fmt: on
                     values = xp.nan_to_num(self.values[..., ::-1])
                     values = num_to_value(values, 1)
                     values = xp.cumprod(values, -1)[..., ::-1]
                     self.values = values * self.nan_triangle
                     values = num_to_value(values, self.get_array_module(values).nan)
+                    # fmt: off
                 elif (
                     self.is_pattern
                     and self.is_additive
                     and (not self.is_disposal_rate)
                 ):  # for additive factor triangles (sum from left to right)
+                    # fmt: on
                     values = xp.nan_to_num(self.values[..., ::-1])
                     values = num_to_value(values, 0)
-                    self.values = (
-                        xp.cumsum(values, -1)[..., ::-1] * self.nan_triangle
-                    )
+                    self.values = (xp.cumsum(values, -1)[..., ::-1] * self.nan_triangle)
                 else:  # for normal value triangles 
-                # also for disposal rate triangles (sum from right to left)
+                    # also for disposal rate triangles (sum from right to left)
                     if self.array_backend not in ["sparse", "dask"]:
                         self.values = (
                             xp.cumsum(xp.nan_to_num(self.values), 3)
@@ -1487,11 +1489,13 @@ def cum_to_incr(self, inplace=False):
         if inplace:
             v = self.valuation_date
             if self.is_cumulative or self.is_cumulative is None:
+                # fmt: off
                 if (
                     self.is_pattern
                     and (not self.is_additive)
                     and (not self.is_disposal_rate)
                 ):  # for multiplicative ldf triangles (div from left to right)
+                    # fmt: on
                     xp = self.get_array_module()
                     self.values = num_to_value(xp.nan_to_num(self.values), 1)
                     diff = self.iloc[..., :-1] / self.iloc[..., 1:].values
@@ -1503,15 +1507,18 @@ def cum_to_incr(self, inplace=False):
                         axis=3,
                     )
                     self.values = self.values * self.nan_triangle
+                    # fmt: fn
                 elif (
                     self.is_pattern
                     and self.is_additive
                     and (not self.is_disposal_rate)
                 ):  # for additive factor triangles (diff from left to right)
+                    # fmt: on
                     diff = self.iloc[..., :-1].values - self.iloc[..., 1:]
+                    diff.development = self.iloc[..., :-1].development
                     self = concat((diff, self.iloc[..., -1]), axis=3)
                 else:  # for normal value triangles 
-                # also for disposal rate triangles (diff from right to left)
+                    # also for disposal rate triangles (diff from right to left)
                     diff = self.iloc[..., 1:] - self.iloc[..., :-1].values
                     self = concat((self.iloc[..., 0], diff), axis=3)
                 self.is_cumulative = False
diff --git a/chainladder/development/tests/test_incremental.py b/chainladder/development/tests/test_incremental.py
index 8c5ef4d2a..539a4fa69 100644
--- a/chainladder/development/tests/test_incremental.py
+++ b/chainladder/development/tests/test_incremental.py
@@ -4,11 +4,6 @@
 import numpy as np
 import pytest
 
-from typing import TYPE_CHECKING
-
-if TYPE_CHECKING:
-    from chainladder import Triangle
-
 
 def test_schmidt():
     tri = cl.load_sample("ia_sample")
@@ -109,8 +104,4 @@ def test_setting_cum(atol: float) -> None:
     )
     orig_zeta_ = ia.zeta_
     ia.cum_zeta_ = ia.cum_zeta_ * 2
-    assert np.allclose(
-        ia.zeta_.values,
-        (orig_zeta_ * 2).values,
-        atol=atol
-    )
+    assert np.allclose(ia.zeta_.values, (orig_zeta_ * 2).values, atol=atol)

From 36c04d893a72aad03624b7dccccf206d60d006d3 Mon Sep 17 00:00:00 2001
From: "henrydingliu@gmail.com" <henrydingliu@gmail.com>
Date: Mon, 21 Sep 2026 17:36:30 +0000
Subject: [PATCH 07/11] bugbot fix

---
 chainladder/core/triangle.py | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/chainladder/core/triangle.py b/chainladder/core/triangle.py
index 891eb6367..083343f09 100644
--- a/chainladder/core/triangle.py
+++ b/chainladder/core/triangle.py
@@ -1514,8 +1514,7 @@ def cum_to_incr(self, inplace=False):
                     and (not self.is_disposal_rate)
                 ):  # for additive factor triangles (diff from left to right)
                     # fmt: on
-                    diff = self.iloc[..., :-1].values - self.iloc[..., 1:]
-                    diff.development = self.iloc[..., :-1].development
+                    diff = self.iloc[..., :-1] - self.iloc[..., 1:].values
                     self = concat((diff, self.iloc[..., -1]), axis=3)
                 else:  # for normal value triangles 
                     # also for disposal rate triangles (diff from right to left)

From 1eae41e3cdcdcbacdd9af2645ff33f8714140cef Mon Sep 17 00:00:00 2001
From: "henrydingliu@gmail.com" <henrydingliu@gmail.com>
Date: Sun, 4 Oct 2026 08:30:09 +0000
Subject: [PATCH 08/11] simplifying code

---
 chainladder/core/triangle.py | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/chainladder/core/triangle.py b/chainladder/core/triangle.py
index 083343f09..62bb6ea5a 100644
--- a/chainladder/core/triangle.py
+++ b/chainladder/core/triangle.py
@@ -1051,9 +1051,7 @@ def is_additive(self) -> bool:
         """
         Indicates whether the Triangle holds a normal additive ratio (which goes from tail to head)
         """
-        if hasattr(self, "_is_additive"):
-            return self._is_additive
-        return False
+        getattr(self, "_is_additive", False)
 
     @is_additive.setter
     def is_additive(self, is_add: bool) -> None:

From c380a4ce79cde31c5779dee417cb078f24f20c2c Mon Sep 17 00:00:00 2001
From: "henrydingliu@gmail.com" <henrydingliu@gmail.com>
Date: Sun, 4 Oct 2026 08:39:27 +0000
Subject: [PATCH 09/11] bugbot fix

---
 chainladder/core/triangle.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/chainladder/core/triangle.py b/chainladder/core/triangle.py
index 62bb6ea5a..ade43f467 100644
--- a/chainladder/core/triangle.py
+++ b/chainladder/core/triangle.py
@@ -1051,7 +1051,7 @@ def is_additive(self) -> bool:
         """
         Indicates whether the Triangle holds a normal additive ratio (which goes from tail to head)
         """
-        getattr(self, "_is_additive", False)
+        return getattr(self, "_is_additive", False)
 
     @is_additive.setter
     def is_additive(self, is_add: bool) -> None:

From 44e2688a7d364d59e4867b4b66d9415d16b18063 Mon Sep 17 00:00:00 2001
From: "henrydingliu@gmail.com" <henrydingliu@gmail.com>
Date: Mon, 5 Oct 2026 22:09:17 +0000
Subject: [PATCH 10/11] improving codecov

---
 chainladder/development/incremental.py        |  3 +--
 .../development/tests/test_incremental.py     | 26 +++++++++++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/chainladder/development/incremental.py b/chainladder/development/incremental.py
index f809bc30b..1b2386deb 100644
--- a/chainladder/development/incremental.py
+++ b/chainladder/development/incremental.py
@@ -27,8 +27,7 @@ def zeta_(self) -> Triangle:
         Gets the estimated incremental additive factors
         """
         if not hasattr(self, "_zeta_"):
-            x = self.__class__.__name__
-            raise AttributeError("'" + x + "' object has no attribute 'zeta_'")
+            raise AttributeError("`zeta_` has not been estimated.")
         return self._zeta_
 
     @zeta_.setter
diff --git a/chainladder/development/tests/test_incremental.py b/chainladder/development/tests/test_incremental.py
index 539a4fa69..6a1c18b09 100644
--- a/chainladder/development/tests/test_incremental.py
+++ b/chainladder/development/tests/test_incremental.py
@@ -97,6 +97,15 @@ def test_pipeline():
 def test_setting_cum(atol: float) -> None:
     """
     IncrementalMixin allows setting cumulative zeta. Validating that the setting function works properly.
+
+    Parameters
+    ----------
+    atol: float
+        the absolute tolerance for this test
+
+    Returns
+    -------
+    None
     """
     tri = cl.load_sample("ia_sample")
     ia = cl.IncrementalAdditive().fit(
@@ -105,3 +114,20 @@ def test_setting_cum(atol: float) -> None:
     orig_zeta_ = ia.zeta_
     ia.cum_zeta_ = ia.cum_zeta_ * 2
     assert np.allclose(ia.zeta_.values, (orig_zeta_ * 2).values, atol=atol)
+
+
+def test_no_zeta(raa: Triangle) -> None:
+    """
+    calling zeta_ on a Triangle without _zeta_ raises
+
+    Parameters
+    ----------
+    raa: Triangle
+        the RAA sample dataset
+
+    Returns
+    -------
+    None
+    """
+    with pytest.raises(AttributeError, match="has not been estimated"):
+        _ = raa.zeta_

From d461b140e3167921af769c8d9b8a1b72d88454d3 Mon Sep 17 00:00:00 2001
From: "henrydingliu@gmail.com" <henrydingliu@gmail.com>
Date: Mon, 5 Oct 2026 22:10:50 +0000
Subject: [PATCH 11/11] ruff fix

---
 chainladder/development/tests/test_incremental.py | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/chainladder/development/tests/test_incremental.py b/chainladder/development/tests/test_incremental.py
index 6a1c18b09..6ff4aa0fe 100644
--- a/chainladder/development/tests/test_incremental.py
+++ b/chainladder/development/tests/test_incremental.py
@@ -4,6 +4,11 @@
 import numpy as np
 import pytest
 
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from chainladder.core import Triangle
+
 
 def test_schmidt():
     tri = cl.load_sample("ia_sample")