Issue 6573: Fix set.union() for cases where self is in the argument chain.
diff --git a/Misc/NEWS b/Misc/NEWS
index b6ccc09..25bbabb 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -14,6 +14,9 @@
 
 - Issue #6540: Fixed crash for bytearray.translate() with invalid parameters.
 
+- Issue #6573: set.union() stopped processing inputs if an instance of self
+  occurred in the argument chain.
+
 - Issue #1616979: Added the cp720 (Arabic DOS) encoding.
 
 - Issue #6070: On posix platforms import no longer copies the execute bit