diff --git a/python/mwmatching.py b/python/mwmatching.py
index 4d33824..f7f61ec 100644
--- a/python/mwmatching.py
+++ b/python/mwmatching.py
@@ -858,6 +858,8 @@ class _MatchingContext:
         This function takes time O(n * log(n)).
         """
 
+        assert not self.scan_queue
+
         # Remove blossom labels and unwind lazy dual updates.
         for blossom in self.trivial_blossom + self.nontrivial_blossom:
             if blossom.parent is None:
@@ -867,9 +869,6 @@ class _MatchingContext:
             assert blossom.label == _LABEL_NONE
             blossom.tree_edge = None
 
-        # Clear the scan queue.
-        self.scan_queue.clear()
-
         # Reset least-slack edge tracking.
         self.lset_reset()
 
@@ -1121,6 +1120,7 @@ class _MatchingContext:
             sub.tree_edge = path_edges[p+1]
 
         # Delete the expanded blossom.
+        # TODO -- list manipulation is too slow
         self.nontrivial_blossom.remove(blossom)
 
     def expand_unlabeled_blossom(self, blossom: _NonTrivialBlossom) -> None:
@@ -1160,6 +1160,7 @@ class _MatchingContext:
                 sub.delta2_node = self.delta2_queue.insert(prio, sub)
 
         # Delete the expanded blossom.
+        # TODO -- list manipulation is too slow
         self.nontrivial_blossom.remove(blossom)
 
     #