diff --git a/python/src/self.py b/python/src/self.py
new file mode 100644
index 0000000..342ada9
--- /dev/null
+++ b/python/src/self.py
@@ -0,0 +1,18 @@
+# Copyright (c) 2024 RenChu Wang - All Rights Reserved
+
+import abc
+from collections.abc import Sequence
+from typing import Protocol, TypeVar
+
+T = TypeVar("T", bound="HasChildren", covariant=True)
+
+
+class HasChildren(Protocol[T]):
+    @abc.abstractmethod
+    def children(self) -> Sequence[T]: ...
+
+
+class HasSelfAsChildren(HasChildren["HasSelfAsChildren"]):
+    # It does autocomplete. Seems fine.
+    def children(self) -> Sequence["HasSelfAsChildren"]:
+        return []