support indexing in CDP
diff --git a/tests/test_x509_ext.py b/tests/test_x509_ext.py
index cfca579..7e46e5a 100644
--- a/tests/test_x509_ext.py
+++ b/tests/test_x509_ext.py
@@ -2938,6 +2938,32 @@
         assert cdp != cdp4
         assert cdp != object()
 
+    def test_indexing(self):
+        ci = x509.CRLDistributionPoints([
+            x509.DistributionPoint(
+                None, None, None,
+                [x509.UniformResourceIdentifier(u"uri://thing")],
+            ),
+            x509.DistributionPoint(
+                None, None, None,
+                [x509.UniformResourceIdentifier(u"uri://thing2")],
+            ),
+            x509.DistributionPoint(
+                None, None, None,
+                [x509.UniformResourceIdentifier(u"uri://thing3")],
+            ),
+            x509.DistributionPoint(
+                None, None, None,
+                [x509.UniformResourceIdentifier(u"uri://thing4")],
+            ),
+            x509.DistributionPoint(
+                None, None, None,
+                [x509.UniformResourceIdentifier(u"uri://thing5")],
+            ),
+        ])
+        assert ci[-1] == ci[4]
+        assert ci[2:6:2] == [ci[2], ci[4]]
+
 
 @pytest.mark.requires_backend_interface(interface=RSABackend)
 @pytest.mark.requires_backend_interface(interface=X509Backend)