Added explanatory comment

ldp/alg/optimizer/ape.py

@@ -193,8 +193,10 @@ def aggregate_trajectory(self, trajectory: Trajectory) -> None:
                 if result is None or not result.messages or not result.prompt:
                     continue
                 # (x: first prompt's user message's content, y: AI response's content)
+                # m is a Message with a result of the LLM. Which completes only strings.
+                # and we checked the result exists above.
                 x = next(
-                    cast(str, m.content)
+                    cast(str, m.content) 
                     for m in result.prompt
                     if (isinstance(m, Message) and m.role == "user")
                 )