Degeneralization keeps track of levels.

* NEWS: Document this.
* spot/twa/twagraph.cc: `copy_state_names_from` handles this new info.
* spot/twaalgos/degen.hh, spot/twaalgos/degen.cc: Implement it.
* tests/python/origstate.py, tests/python/simstate.py: Update tests to
  reflect the change.

spot/twa/twagraph.cc

@@ -607,6 +607,19 @@ namespace spot
           }
         os->resize(used_states);
       }
+    if (auto dl = get_named_prop<std::vector<unsigned>>("degen-levels"))
+      {
+        unsigned size = dl->size();
+        for (unsigned s = 0; s < size; ++s)
+          {
+            unsigned dst = newst[s];
+            if (dst == s || dst == -1U)
+              continue;
+            assert(dst < s);
+            (*dl)[dst] = (*dl)[s];
+          }
+        dl->resize(used_states);
+      }
     init_number_ = newst[init_number_];
     g_.defrag_states(std::move(newst), used_states);
   }
@@ -637,8 +650,11 @@ namespace spot
       return;
 
     auto orig = get_named_prop<std::vector<unsigned>>("original-states");
+    auto lvl = get_named_prop<std::vector<unsigned>>("degen-levels");
     auto sims = get_named_prop<std::vector<unsigned>>("simulated-states");
 
+    assert(!lvl || orig);
+
     if (orig && sims)
       throw std::runtime_error("copy_state_names_from(): original-states and "
                                "simulated-states are both set");
@@ -646,6 +662,9 @@ namespace spot
     if (orig && orig->size() != num_states())
       throw std::runtime_error("copy_state_names_from(): unexpected size "
                                "for original-states");
+    if (lvl && lvl->size() != num_states())
+      throw std::runtime_error("copy_state_names_from(): unexpected size "
+                               "for degen-levels");
 
     if (sims && sims->size() != other->num_states())
       throw std::runtime_error("copy_state_names_from(): unexpected size "
@@ -677,6 +696,8 @@ namespace spot
               throw std::runtime_error("copy_state_names_from(): state does not"
                                        " exist in source automaton");
             newname = other->format_state(other_s);
+            if (lvl)
+              newname += '#' + std::to_string((*lvl)[s]);
           }
         names->emplace_back(newname);
       }

spot/twaalgos/degen.cc

@@ -240,8 +240,11 @@ namespace spot
       res->prop_copy(a, { false, true, true, true, true, true });
 
       auto orig_states = new std::vector<unsigned>();
+      auto levels = new std::vector<unsigned>();
       orig_states->reserve(a->num_states()); // likely more are needed.
+      levels->reserve(a->num_states());
       res->set_named_prop("original-states", orig_states);
+      res->set_named_prop("degen-levels", levels);
 
       // Create an order of acceptance conditions.  Each entry in this
       // vector correspond to an acceptance set.  Each index can
@@ -328,6 +331,7 @@ namespace spot
 
           assert(ns == orig_states->size());
           orig_states->emplace_back(ds.first);
+          levels->emplace_back(ds.second);
 
           // Level cache stores one encountered level for each state
           // (the value of use_lvl_cache determinates which level

spot/twaalgos/degen.hh

@@ -56,6 +56,9 @@ namespace spot
   /// to retrieve it.  Note that these functions may return the original
   /// automaton as-is if it is already degeneralized; in this case
   /// the "original-states" property is not defined.
+  /// Similarly, the property "degen-levels" keeps track of the degeneralization
+  /// levels.  To retrieve it, call
+  /// `aut->get_named_prop<std::vector<unsigned>>("degen-levels")`.
   /// \@{
   SPOT_API twa_graph_ptr
   degeneralize(const const_twa_graph_ptr& a, bool use_z_lvl = true,