Index: Objects/setobject.c
===================================================================
--- Objects/setobject.c	(revision 81048)
+++ Objects/setobject.c	(working copy)
@@ -1476,9 +1476,25 @@
         setentry *entry;
         Py_ssize_t pos = 0;
 
-        while (set_next((PySetObject *)other, &pos, &entry))
-            if (set_discard_entry(so, entry) == -1)
-                return -1;
+	if (PySet_GET_SIZE(so) < PySet_GET_SIZE(other) ) {
+	    while (set_next(so, &pos, &entry)) {
+		int rv = set_contains_entry((PySetObject *)other, entry);
+		if (rv == -1)
+		    return -1;
+		if (rv == 1) {
+		    PyObject *old_key;
+		    old_key = entry->key;
+		    Py_INCREF(dummy);
+		    entry->key = dummy;
+		    so->used--;
+		    Py_DECREF(old_key);
+		}
+	    }
+	} else {
+	    while (set_next((PySetObject *)other, &pos, &entry))
+		if (set_discard_entry(so, entry) == -1)
+		    return -1;
+	}
     } else {
         PyObject *key, *it;
         it = PyObject_GetIter(other);