--- a
+++ b/tests/test_base.py
@@ -0,0 +1,17 @@
+import pytest
+from deidentify.base import Annotation
+
+
+def test_annotation():
+    ann_a = Annotation(text='test', start=12, end=15, tag='ABC', doc_id='123', ann_id='456')
+    ann_b = Annotation(text='test', start=12, end=15, tag='ABC', doc_id='123', ann_id='456')
+    ann_c = Annotation(text='test2', start=12, end=15, tag='ABC', doc_id='123', ann_id='456')
+
+    assert ann_a == ann_b
+    assert ann_a != ann_c
+
+    with pytest.raises(AttributeError):
+        ann_a.text = "Annotation should be immutable"
+
+    # Annotation should also be hashable
+    assert len(set([ann_a, ann_b, ann_c])) == 2