diff --git a/15-more-types/petbox/petbox.py b/15-more-types/petbox/petbox.py
index 04b72ea..5b963f3 100644
--- a/15-more-types/petbox/petbox.py
+++ b/15-more-types/petbox/petbox.py
@@ -1,3 +1,8 @@
+"""
+Example adapted from `Atomic Kotlin` by Bruce Eckel & Svetlana Isakova,
+chapter `Creating Generics`, section `Variance`.
+"""
+
 from typing import TypeVar, Generic, Any
 
 
diff --git a/15-more-types/petbox/petbox_demo.py b/15-more-types/petbox/petbox_demo.py
index d09cab4..a7e8bab 100644
--- a/15-more-types/petbox/petbox_demo.py
+++ b/15-more-types/petbox/petbox_demo.py
@@ -1,3 +1,8 @@
+"""
+Example adapted from `Atomic Kotlin` by Bruce Eckel & Svetlana Isakova,
+chapter `Creating Generics`, section `Variance`.
+"""
+
 from typing import TYPE_CHECKING
 
 from petbox import *
@@ -11,7 +16,7 @@ cat_box.put(si)
 
 animal = cat_box.get()
 
-#if TYPE_CHECKING:
+# if TYPE_CHECKING:
 #    reveal_type(animal)  # Revealed: petbox.Cat*