diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4801cad..f19fe51 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,24 @@
 
 All notable changes to this project will be documented in this file.
 
+## [0.2.3] - 2024-12-05
+
+### 🚀 Features
+
+- Add `central` to shortname columns
+
+### 🐛 Bug Fixes
+
+- `&` and `|` with `None` return original `Q`. Previously, `Q(...) | None` would return a query that evaluated to `True` everywhere.
+
+### 📚 Documentation
+
+- List defined operators on `Q` (`&`, `|`, `~`, `==`) in the docstring of `CombineQMixin`.
+
+### 🧪 Testing
+
+- ensure that `&` and `|` with `None` return original `Q`.
+
 ## [0.2.2] - 2024-12-03
 
 ### 🚀 Features
@@ -202,6 +220,7 @@ Initial implementation of the lyDATA library.
 <!-- generated by git-cliff -->
 <!-- markdownlint-disable-file MD024 -->
 
+[0.2.3]: https://github.com/rmnldwg/lydata/compare/0.2.2..0.2.3
 [0.2.2]: https://github.com/rmnldwg/lydata/compare/0.2.1..0.2.2
 [0.2.1]: https://github.com/rmnldwg/lydata/compare/0.2.0..0.2.1
 [0.2.0]: https://github.com/rmnldwg/lydata/compare/0.1.2..0.2.0
diff --git a/lydata/accessor.py b/lydata/accessor.py
index d41bd0a..4efbf84 100644
--- a/lydata/accessor.py
+++ b/lydata/accessor.py
@@ -55,17 +55,34 @@ def _get_all_true(df: pd.DataFrame) -> pd.Series:
 
 
 class CombineQMixin:
-    """Mixin class for combining queries."""
+    """Mixin class for combining queries.
+
+    Four operators are defined for combining queries:
+
+    1. ``&`` for logical AND operations.
+        The returned object is an :py:class:`AndQ` instance and - when executed -
+        returns a boolean mask where both queries are satisfied. When the right-hand
+        side is ``None``, the left-hand side query object is returned unchanged.
+    2. ``|`` for logical OR operations.
+        The returned object is an :py:class:`OrQ` instance and - when executed -
+        returns a boolean mask where either query is satisfied. When the right-hand
+        side is ``None``, the left-hand side query object is returned unchanged.
+    3. ``~`` for inverting a query.
+        The returned object is a :py:class:`NotQ` instance and - when executed -
+        returns a boolean mask where the query is not satisfied.
+    4. ``==`` for checking if two queries are equal.
+        Two queries are equal if their column names, operators, and values are equal.
+        Note that this does not check if the queries are semantically equal, i.e., if
+        they would return the same result when executed.
+    """
 
     def __and__(self, other: QTypes | None) -> AndQ:
         """Combine two queries with a logical AND."""
-        other = other or NoneQ()
-        return AndQ(self, other)
+        return self if other is None else AndQ(self, other)
 
     def __or__(self, other: QTypes | None) -> OrQ:
         """Combine two queries with a logical OR."""
-        other = other or NoneQ()
-        return OrQ(self, other)
+        return self if other is None else OrQ(self, other)
 
     def __invert__(self) -> NotQ:
         """Negate the query."""
@@ -166,6 +183,8 @@ class AndQ(CombineQMixin):
     1     True
     2    False
     dtype: bool
+    >>> all((q1 & None).execute(df) == q1.execute(df))
+    True
     """
 
     def __init__(self, q1: QTypes, q2: QTypes) -> None:
@@ -198,6 +217,8 @@ class OrQ(CombineQMixin):
     1    False
     2     True
     Name: col1, dtype: bool
+    >>> all((q1 | None).execute(df) == q1.execute(df))
+    True
     """
 
     def __init__(self, q1: QTypes, q2: QTypes) -> None:
@@ -257,6 +278,7 @@ def execute(self, df: pd.DataFrame) -> pd.Series:
 
 
 QTypes = Q | AndQ | OrQ | NotQ | None
+"""Type for a query object or a combination of query objects."""
 
 
 class C:
diff --git a/lydata/utils.py b/lydata/utils.py
index 5b2cdb0..fcb4528 100644
--- a/lydata/utils.py
+++ b/lydata/utils.py
@@ -120,6 +120,7 @@ def get_default_column_map() -> _ColumnMap:
             _ColumnSpec("midext", ("tumor", "1", "extension")),
             _ColumnSpec("subsite", ("tumor", "1", "subsite")),
             _ColumnSpec("volume", ("tumor", "1", "volume")),
+            _ColumnSpec("central", ("tumor", "1", "central")),
         ]
     )