ch11-24: clean up by @eumiro & sync with Atlas

13-protocol-abc/double/double_protocol.py

@@ -0,0 +1,11 @@
+from typing import TypeVar, Protocol
+
+T = TypeVar('T')  # <1>
+
+class Repeatable(Protocol):
+    def __mul__(self: T, repeat_count: int) -> T: ...  # <2>
+
+RT = TypeVar('RT', bound=Repeatable)  # <3>
+
+def double(x: RT) -> RT:  # <4>
+    return x * 2