Issue #9566: explain why (int)len cannot underflow
diff --git a/Modules/pyexpat.c b/Modules/pyexpat.c
index dea682d..21ab088 100644
--- a/Modules/pyexpat.c
+++ b/Modules/pyexpat.c
@@ -839,6 +839,7 @@
 finally:
     Py_XDECREF(arg);
     Py_XDECREF(str);
+    /* len <= buf_size <= INT_MAX (see above) */
     return (int)len;
 }