#3021: Antoine Pitrou's Lexical exception handlers
diff --git a/Include/frameobject.h b/Include/frameobject.h
index d2afe8b..65ebd2a 100644
--- a/Include/frameobject.h
+++ b/Include/frameobject.h
@@ -27,13 +27,13 @@
PyObject **f_stacktop;
PyObject *f_trace; /* Trace function */
- /* If an exception is raised in this frame, the next three are used to
- * record the exception info (if any) originally in the thread state. See
- * comments before set_exc_info() -- it's not obvious.
- * Invariant: if _type is NULL, then so are _value and _traceback.
- * Desired invariant: all three are NULL, or all three are non-NULL. That
- * one isn't currently true, but "should be".
- */
+ /* In a generator, we need to be able to swap between the exception
+ state inside the generator and the exception state of the calling
+ frame (which shouldn't be impacted when the generator "yields"
+ from an except handler).
+ These three fields exist exactly for that, and are unused for
+ non-generator frames. See the SAVE_EXC_STATE and SWAP_EXC_STATE
+ macros in ceval.c for details of their use. */
PyObject *f_exc_type, *f_exc_value, *f_exc_traceback;
PyThreadState *f_tstate;