From a07446808a0722997aa70820195f5c16a525a713 Mon Sep 17 00:00:00 2001
From: jan <jan@orange.mpxd.net>
Date: Fri, 7 Apr 2023 16:55:50 -0700
Subject: [PATCH] should be union; we want to exclude dangling refs

---
 masque/library.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/masque/library.py b/masque/library.py
index c4cb41e..f9411b1 100644
--- a/masque/library.py
+++ b/masque/library.py
@@ -781,7 +781,7 @@ class MutableLibrary(Library, MutableMapping[str, 'Pattern'], metaclass=ABCMeta)
         keep |= set(tops)
 
         new = type(self)()
-        for key in keep - set(self.keys()):
+        for key in keep & set(self.keys()):
             new._merge(key, self, key)
         return new