Collapse double negation in NotFilter

eland/filter.py

@@ -90,8 +90,16 @@ def __init__(self, *args: BooleanFilter) -> None:
 class NotFilter(BooleanFilter):
     def __init__(self, x: BooleanFilter) -> None:
         super().__init__()
+        self._x = x
         self._filter = {"bool": {"must_not": x.build()}}
 
+    def __invert__(self) -> "BooleanFilter":
+        # Double negation cancels out: ``~~filter`` is equivalent to ``filter``,
+        # so return the wrapped filter instead of nesting another ``must_not``.
+        # This only applies to ``NotFilter`` itself; leaf filters that merely
+        # use ``must_not`` internally (e.g. ``IsNull``) are unaffected.
+        return self._x
+
 
 # LeafBooleanFilter
 class GreaterEqual(BooleanFilter):

tests/operators/test_operators_pytest.py

@@ -129,10 +129,21 @@ def test_not_filter(self):
         assert exp.build() == {"bool": {"must_not": {"range": {"a": {"gte": 2}}}}}
 
     def test_not_not_filter(self):
+        # Double negation cancels out: ~~filter is equivalent to filter.
         exp = ~~GreaterEqual("a", 2)
+        assert exp.build() == {"range": {"a": {"gte": 2}}}
 
+    def test_not_not_not_filter(self):
+        # An odd number of negations collapses to a single negation.
+        exp = ~~~GreaterEqual("a", 2)
+        assert exp.build() == {"bool": {"must_not": {"range": {"a": {"gte": 2}}}}}
+
+    def test_not_isnull_not_collapsed(self):
+        # IsNull uses must_not internally but is not a NotFilter, so inverting
+        # it must not be treated as a double negation.
+        exp = ~IsNull("a")
         assert exp.build() == {
-            "bool": {"must_not": {"bool": {"must_not": {"range": {"a": {"gte": 2}}}}}}
+            "bool": {"must_not": {"bool": {"must_not": {"exists": {"field": "a"}}}}}
         }
 
     def test_not_and_filter(self):