diff --git a/NEWS b/NEWS
index 03dd5bd3a..c55eb1513 100644
--- a/NEWS
+++ b/NEWS
@@ -189,6 +189,13 @@ New in spot 2.9.6.dev (not yet released)
   - Some formulas using ->, <->, or xor were not properly detected as
     purely universal or pure eventualities.
 
+  Deprecation notices:
+
+  - twa_graph::defrag_states() and digraph::defrag_states() now take
+    the renaming vector by reference instead of rvalue reference.  The
+    old prototype is still supported but will emit a deprecation
+    warning.
+
 New in spot 2.9.6 (2021-01-18)
 
   Build:
diff --git a/spot/graph/graph.hh b/spot/graph/graph.hh
index 507bc849e..efb83abf0 100644
--- a/spot/graph/graph.hh
+++ b/spot/graph/graph.hh
@@ -1302,7 +1302,9 @@ namespace spot
     ///
     /// \param used_states the number of states used (after
     /// renumbering)
-    void defrag_states(std::vector<unsigned>&& newst, unsigned used_states)
+    ///
+    ///@{
+    void defrag_states(const std::vector<unsigned>& newst, unsigned used_states)
     {
       SPOT_ASSERT(newst.size() >= states_.size());
       SPOT_ASSERT(used_states > 0);
@@ -1368,5 +1370,13 @@ namespace spot
       //std::cerr << "\nafter defrag\n";
       //dump_storage(std::cerr);
     }
+
+    // prototype was changed in Spot 2.10
+    SPOT_DEPRECATED("use reference version of this method")
+    void defrag_states(std::vector<unsigned>&& newst, unsigned used_states)
+    {
+      return defrag_states(newst, used_states);
+    }
+    ///@}
   };
 }
diff --git a/spot/parseaut/parseaut.yy b/spot/parseaut/parseaut.yy
index 12a7a0e5a..f42f92876 100644
--- a/spot/parseaut/parseaut.yy
+++ b/spot/parseaut/parseaut.yy
@@ -1,5 +1,5 @@
 /* -*- coding: utf-8 -*-
-** Copyright (C) 2014-2020 Laboratoire de Recherche et Développement
+** Copyright (C) 2014-2021 Laboratoire de Recherche et Développement
 ** de l'Epita (LRDE).
 **
 ** This file is part of Spot, a model checking library.
@@ -2077,7 +2077,7 @@ lbtt: lbtt-header lbtt-body ENDAUT
 	    assert(s == (unsigned) res.states);
 	    for (auto& i: res.start)
 	      i.second.front() = rename[i.second.front()];
-	    res.h->aut->get_graph().defrag_states(std::move(rename), s);
+	    res.h->aut->get_graph().defrag_states(rename, s);
 	  }
 	 res.info_states.resize(res.h->aut->num_states());
 	 for (auto& s: res.info_states)
diff --git a/spot/twa/twagraph.cc b/spot/twa/twagraph.cc
index 68cbd289d..c19585b69 100644
--- a/spot/twa/twagraph.cc
+++ b/spot/twa/twagraph.cc
@@ -328,7 +328,7 @@ namespace spot
       else
         s = -1U;
 
-    defrag_states(std::move(remap), st);
+    defrag_states(remap, st);
   }
 
   void twa_graph::purge_unreachable_states(shift_action* f, void* action_data)
@@ -383,7 +383,7 @@ namespace spot
     if (f)
       (*f)(todo, action_data);
 
-    defrag_states(std::move(todo), current);
+    defrag_states(todo, current);
   }
 
   void twa_graph::purge_dead_states()
@@ -580,13 +580,13 @@ namespace spot
     if (prop_complete().is_false())
       prop_complete(trival::maybe());
 
-    defrag_states(std::move(useful), current);
+    defrag_states(useful, current);
 
     if (purge_unreachable_needed)
       purge_unreachable_states();
   }
 
-  void twa_graph::defrag_states(std::vector<unsigned>&& newst,
+  void twa_graph::defrag_states(std::vector<unsigned>& newst,
                                 unsigned used_states)
   {
     if (!is_existential())
@@ -660,7 +660,7 @@ namespace spot
                   }
                 else
                   {
-                    // register this new destination group, add et two
+                    // register this new destination group, add it to
                     // newst, and use the index in newst to relabel
                     // the state so that graph::degrag_states() will
                     // eventually update it to the correct value.
@@ -738,7 +738,7 @@ namespace spot
           }
       }
     init_number_ = newst[init_number_];
-    g_.defrag_states(std::move(newst), used_states);
+    g_.defrag_states(newst, used_states);
   }
 
   void twa_graph::remove_unused_ap()
diff --git a/spot/twa/twagraph.hh b/spot/twa/twagraph.hh
index b8e248916..072a0d04c 100644
--- a/spot/twa/twagraph.hh
+++ b/spot/twa/twagraph.hh
@@ -689,6 +689,7 @@ namespace spot
                         dests2.begin());
     }
 
+#ifndef SWIG
     /// \brief Renumber all states, and drop some.
     ///
     /// This semi-internal function is a wrapper around
@@ -705,11 +706,25 @@ namespace spot
     /// \param newst A vector indicating how each state should be
     /// renumbered.  Use -1U to mark an unreachable state for removal.
     /// Ignoring the occurrences of -1U, the renumbering is expected
-    /// to satisfy newst[i] ≤ i for all i.
+    /// to satisfy newst[i] ≤ i for all i.   If the automaton contains
+    /// universal branching, this vector is likely to be modified by
+    /// this function, so do not reuse it afterwards.
     ///
     /// \param used_states the number of states used after
     /// renumbering.
-    void defrag_states(std::vector<unsigned>&& newst, unsigned used_states);
+    ///@{
+    void defrag_states(std::vector<unsigned>& newst,
+                       unsigned used_states);
+
+    // prototype was changed in Spot 2.10
+    SPOT_DEPRECATED("use reference version of this method")
+    void defrag_states(std::vector<unsigned>&& newst,
+                       unsigned used_states)
+    {
+      return defrag_states(newst, used_states);
+    }
+    ///@}
+#endif // SWIG
 
     /// \brief Print the data structures used to represent the
     /// automaton in dot's format.