diff --git a/spot/twa/twagraph.cc b/spot/twa/twagraph.cc
index c2bdd5650..2fd2eb070 100644
--- a/spot/twa/twagraph.cc
+++ b/spot/twa/twagraph.cc
@@ -372,13 +372,12 @@ namespace spot
       throw std::runtime_error(
           "twa_graph::merge_states() does not work on alternating automata");
 
+    const unsigned nthreads = get_nthreads();
+
     typedef graph_t::edge_storage_t tr_t;
-    g_.sort_edges_([](const tr_t& lhs, const tr_t& rhs)
+    g_.sort_edges_srcfirst_([](const tr_t& lhs, const tr_t& rhs)
     {
-      if (lhs.src < rhs.src)
-        return true;
-      if (lhs.src > rhs.src)
-        return false;
+      assert(lhs.src == rhs.src);
       if (lhs.acc < rhs.acc)
         return true;
       if (lhs.acc > rhs.acc)
@@ -449,7 +448,7 @@ namespace spot
     // Represents which states share a hash
     // Head is in the unordered_map,
     // hash_linked_list is like a linked list structure
-    // of false pointers
+    // of fake pointers
 
     auto hash_linked_list = std::vector<unsigned>(n_states, -1u);
     auto s_to_hash = std::vector<size_t>(n_states, 0);
@@ -530,8 +529,8 @@ namespace spot
           };
 
 
-        static auto checked1 = std::vector<char>();
-        static auto checked2 = std::vector<char>();
+        thread_local auto checked1 = std::vector<char>();
+        thread_local auto checked2 = std::vector<char>();
 
         auto [i1, nsl1, sl1, e1] = e_idx[s1];
         auto [i2, nsl2, sl2, e2] = e_idx[s2];
@@ -585,12 +584,10 @@ namespace spot
             // More efficient version?
             // Skip checked edges
             // Last element serves as break
-            for (; checked1[idx1 - i1]; ++idx1)
-              {
-              }
-            for (; checked2[idx2 - i2]; ++idx2)
-              {
-              }
+            while (checked1[idx1 - i1])
+              ++idx1;
+            while (checked2[idx2 - i2])
+              ++idx2;
             // If one is out of bounds, so is the other
             if (idx1 == e1)
               {
@@ -614,47 +611,107 @@ namespace spot
     const unsigned nb_states = num_states();
     std::vector<unsigned> remap(nb_states, -1U);
 
-    for (unsigned i = 0; i != nb_states; ++i)
+    // Check each hash
+    auto check_ix = [&](unsigned ix)
       {
-        auto j = spe && (*sp)[i] ? player_map.at(s_to_hash[i]).first
-                                 : env_map.at(s_to_hash[i]).first;
-        for (; j<i; j=hash_linked_list[j])
-          {
-            if (state_equal(j, i))
-              {
-                remap[i] = (remap[j] != -1U) ? remap[j] : j;
+        // Reduce cache miss
+        thread_local auto v = std::vector<unsigned>();
+        v.clear();
+        for (auto i = ix; i != -1U; i = hash_linked_list[i])
+          v.push_back(i);
+        const unsigned N = v.size();
 
-                // Because of the special self-loop tests we use above,
-                // it's possible that i can be mapped to remap[j] even
-                // if j was last compatible states found.  Consider the
-                // following cases, taken from an actual test case:
-                // 18 is equal to 5, 35 is equal to 18, but 35 is not
-                // equal to 5.
-                //
-                // State: 5
-                // [0&1&2] 8 {3}
-                // [!0&1&2] 10 {1}
-                // [!0&!1&!2] 18 {1}
-                // [!0&!1&2] 19 {1}
-                // [!0&1&!2] 20 {1}
-                //
-                // State: 18
-                // [0&1&2] 8 {3}
-                // [!0&1&2] 10 {1}
-                // [!0&!1&!2] 18 {1} // self-loop
-                // [!0&!1&2] 19 {1}
-                // [!0&1&!2] 20 {1}
-                //
-                // State: 35
-                // [0&1&2] 8 {3}
-                // [!0&1&2] 10 {1}
-                // [!0&!1&!2] 35 {1} // self-loop
-                // [!0&!1&2] 19 {1}
-                // [!0&1&!2] 20 {1}
-                break;
+        for (unsigned idx = 0; idx < N; ++idx)
+          {
+            auto i = v[idx];
+            for (unsigned jdx = 0; jdx < idx; ++jdx)
+              {
+                auto j = v[jdx];
+                if (state_equal(j, i))
+                  {
+                    remap[i] = (remap[j] != -1U) ? remap[j] : j;
+
+                    // Because of the special self-loop tests we use above,
+                    // it's possible that i can be mapped to remap[j] even
+                    // if j was last compatible states found.  Consider the
+                    // following cases, taken from an actual test case:
+                    // 18 is equal to 5, 35 is equal to 18, but 35 is not
+                    // equal to 5.
+                    //
+                    // State: 5
+                    // [0&1&2] 8 {3}
+                    // [!0&1&2] 10 {1}
+                    // [!0&!1&!2] 18 {1}
+                    // [!0&!1&2] 19 {1}
+                    // [!0&1&!2] 20 {1}
+                    //
+                    // State: 18
+                    // [0&1&2] 8 {3}
+                    // [!0&1&2] 10 {1}
+                    // [!0&!1&!2] 18 {1} // self-loop
+                    // [!0&!1&2] 19 {1}
+                    // [!0&1&!2] 20 {1}
+                    //
+                    // State: 35
+                    // [0&1&2] 8 {3}
+                    // [!0&1&2] 10 {1}
+                    // [!0&!1&!2] 35 {1} // self-loop
+                    // [!0&!1&2] 19 {1}
+                    // [!0&1&!2] 20 {1}
+                    break;
+                  }
               }
           }
-      }
+      };
+
+    auto upd = [](auto& b, const auto&e, unsigned it)
+      {
+        while ((it > 0) & (b != e))
+          {
+            --it;
+            ++b;
+          }
+      };
+
+    auto worker = [&upd, check_ix, nthreads](unsigned pid, auto begp, auto endp,
+                                             auto bege, auto ende)
+      {
+        upd(begp, endp, pid);
+        upd(bege, ende, pid);
+        for (; begp != endp; upd(begp, endp, nthreads))
+          check_ix(begp->second.first);
+        for (; bege != ende; upd(bege, ende, nthreads))
+          check_ix(bege->second.first);
+      };
+
+    {
+      auto begp = player_map.begin();
+      auto endp = player_map.end();
+      auto bege = env_map.begin();
+      auto ende = env_map.end();
+
+
+      if ((nthreads == 1) & (num_states() > 1000)) // Bound?
+        {
+          worker(0, begp, endp, bege, ende);
+        }
+      else
+        {
+          static auto tv = std::vector<std::thread>();
+          assert(tv.empty());
+          tv.resize(nthreads);
+          for (unsigned pid = 0; pid < nthreads; ++pid)
+            tv[pid] = std::thread(
+              [worker, pid, begp, endp, bege, ende]()
+              {
+                worker(pid, begp, endp, bege, ende);
+                return;
+              });
+          for (auto& t : tv)
+            t.join();
+          tv.clear();
+        }
+    }
 
     for (auto& e: edges())
       if (remap[e.dst] != -1U)
@@ -765,7 +822,7 @@ namespace spot
         comp_classes_.clear();
         // get all compatible classes
         // Candidate classes share a hash
-        // A state is compatible to a class if it is compatble
+        // A state is compatible to a class if it is compatible
         // to any of its states
         auto& cand_classes = equiv_class_[hi];
         unsigned n_c_classes = cand_classes.size();
diff --git a/tests/python/mergedge.py b/tests/python/mergedge.py
index 4e97abe23..2be4d4984 100644
--- a/tests/python/mergedge.py
+++ b/tests/python/mergedge.py
@@ -23,148 +23,151 @@ import spot
 from unittest import TestCase
 tc = TestCase()
 
-aut = spot.automaton("""HOA: v1 States: 1 Start: 0 AP: 1 "a"
-Acceptance: 1 Inf(0) --BODY-- State: 0 [0] 0 [0] 0 {0} --END--""")
-tc.assertEqual(aut.num_edges(), 2)
-aut.merge_edges()
-tc.assertEqual(aut.num_edges(), 1)
+for nthread in range(1, 16, 2):
+    spot.set_nthreads(nthread)
+    tc.assertEqual(spot.get_nthreads(), nthread)
+    aut = spot.automaton("""HOA: v1 States: 1 Start: 0 AP: 1 "a"
+    Acceptance: 1 Inf(0) --BODY-- State: 0 [0] 0 [0] 0 {0} --END--""")
+    tc.assertEqual(aut.num_edges(), 2)
+    aut.merge_edges()
+    tc.assertEqual(aut.num_edges(), 1)
 
-aut = spot.automaton("""
-HOA: v1
-States: 2
-Start: 0
-AP: 2 "p0" "p1"
-acc-name: Buchi
-Acceptance: 1 Inf(0)
-properties: trans-labels explicit-labels trans-acc complete
---BODY--
-State: 0
-[!0] 0 {0}
-[0] 1 {0}
-State: 1
-[!0&!1] 0 {0}
-[0 | 1] 1
-[0&!1] 1 {0}
---END--""")
-tc.assertEqual(aut.num_edges(), 5)
-aut.merge_edges()
-tc.assertEqual(aut.num_edges(), 5)
-tc.assertFalse(spot.is_deterministic(aut))
-aut = spot.split_edges(aut)
-tc.assertEqual(aut.num_edges(), 9)
-aut.merge_edges()
-tc.assertEqual(aut.num_edges(), 5)
-tc.assertTrue(spot.is_deterministic(aut))
+    aut = spot.automaton("""
+    HOA: v1
+    States: 2
+    Start: 0
+    AP: 2 "p0" "p1"
+    acc-name: Buchi
+    Acceptance: 1 Inf(0)
+    properties: trans-labels explicit-labels trans-acc complete
+    --BODY--
+    State: 0
+    [!0] 0 {0}
+    [0] 1 {0}
+    State: 1
+    [!0&!1] 0 {0}
+    [0 | 1] 1
+    [0&!1] 1 {0}
+    --END--""")
+    tc.assertEqual(aut.num_edges(), 5)
+    aut.merge_edges()
+    tc.assertEqual(aut.num_edges(), 5)
+    tc.assertFalse(spot.is_deterministic(aut))
+    aut = spot.split_edges(aut)
+    tc.assertEqual(aut.num_edges(), 9)
+    aut.merge_edges()
+    tc.assertEqual(aut.num_edges(), 5)
+    tc.assertTrue(spot.is_deterministic(aut))
 
-aut = spot.automaton("""
-HOA: v1
-States: 3
-Start: 0
-AP: 1 "a"
-acc-name: Buchi
-Acceptance: 1 Inf(0)
-properties: trans-labels explicit-labels trans-acc complete
---BODY--
-State: 0
-[!0] 1 {0}
-[0] 2 {0}
-State: 1
-[!0] 1 {0}
-[0] 1
-State: 2
-[!0] 2 {0}
-[0] 1
---END--""")
-aut.merge_states()
-tc.assertEqual(aut.num_edges(), 4)
-tc.assertEqual(aut.num_states(), 2)
-tc.assertTrue(spot.is_deterministic(aut))
-tc.assertTrue(aut.prop_complete())
-aut.merge_states()
-tc.assertEqual(aut.num_edges(), 4)
-tc.assertEqual(aut.num_states(), 2)
-tc.assertTrue(spot.is_deterministic(aut))
-tc.assertTrue(aut.prop_complete())
+    aut = spot.automaton("""
+    HOA: v1
+    States: 3
+    Start: 0
+    AP: 1 "a"
+    acc-name: Buchi
+    Acceptance: 1 Inf(0)
+    properties: trans-labels explicit-labels trans-acc complete
+    --BODY--
+    State: 0
+    [!0] 1 {0}
+    [0] 2 {0}
+    State: 1
+    [!0] 1 {0}
+    [0] 1
+    State: 2
+    [!0] 2 {0}
+    [0] 1
+    --END--""")
+    aut.merge_states()
+    tc.assertEqual(aut.num_edges(), 4)
+    tc.assertEqual(aut.num_states(), 2)
+    tc.assertTrue(spot.is_deterministic(aut))
+    tc.assertTrue(aut.prop_complete())
+    aut.merge_states()
+    tc.assertEqual(aut.num_edges(), 4)
+    tc.assertEqual(aut.num_states(), 2)
+    tc.assertTrue(spot.is_deterministic(aut))
+    tc.assertTrue(aut.prop_complete())
 
 
-aa = spot.automaton("""
-HOA: v1 States: 41 Start: 0 AP: 3 "allfinished" "finished_0"
-"finished_1" acc-name: parity max odd 4 Acceptance: 4 Inf(3) | (Fin(2)
-& (Inf(1) | Fin(0))) properties: trans-labels explicit-labels
-trans-acc colored properties: deterministic --BODY-- State: 0
-[!0&!1&!2] 1 {1} [!0&!1&2] 2 {1} [!0&1&!2] 3 {1} [!0&1&2] 4 {1}
-[0&!1&!2] 5 {1} [0&!1&2] 6 {1} [0&1&!2] 7 {1} [0&1&2] 8 {1} State: 1
-[!0&!1&!2] 1 {1} [!0&!1&2] 2 {1} [0&1&!2] 7 {1} [0&1&2] 8 {1}
-[!0&1&!2] 9 {1} [!0&1&2] 10 {1} [0&!1&!2] 11 {1} [0&!1&2] 12 {1}
-State: 2 [!0&!1&2] 2 {1} [0&1&!2] 7 {1} [0&1&2] 8 {1} [!0&1&2] 10 {1}
-[0&!1&!2] 11 {1} [0&!1&2] 12 {1} [!0&!1&!2] 13 {1} [!0&1&!2] 14 {1}
-State: 3 [0&1&!2] 7 {1} [0&1&2] 8 {1} [!0&1&!2] 9 {1} [!0&1&2] 10 {1}
-[0&!1&!2] 11 {1} [0&!1&2] 12 {1} [!0&!1&!2] 15 {1} [!0&!1&2] 16 {1}
-State: 4 [0&1&!2] 7 {1} [0&1&2] 8 {1} [!0&1&2] 10 {1} [0&!1&!2] 11 {1}
-[0&!1&2] 12 {1} [!0&1&!2] 14 {1} [!0&!1&2] 16 {1} [!0&!1&!2] 17 {1}
-State: 5 [0&1&2] 8 {3} [!0&1&2] 10 {1} [!0&!1&!2] 18 {1} [!0&!1&2] 19
-{1} [!0&1&!2] 20 {1} State: 6 [0&1&2] 8 {1} [!0&1&2] 10 {1} [!0&!1&2]
-19 {1} [!0&1&!2] 20 {1} [!0&!1&!2] 21 {1} State: 7 [0&1&2] 8 {3}
-[!0&1&2] 10 {1} [!0&!1&2] 19 {1} [!0&1&!2] 20 {1} [!0&!1&!2] 22 {1}
-State: 8 [!0&!1&!2] 5 {1} [0&1&2] 8 {3} [!0&1&2] 10 {1} [!0&!1&2] 19
-{1} [!0&1&!2] 20 {1} State: 9 [0&!1&!2] 5 {1} [0&1&!2] 7 {1} [0&1&2] 8
-{1} [!0&1&!2] 9 {1} [!0&1&2] 10 {1} [0&!1&2] 12 {1} [!0&!1&!2] 23 {1}
-[!0&!1&2] 24 {1} State: 10 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2] 8
-{3} [!0&1&2] 10 {2} [0&!1&2] 12 {3} [!0&1&!2] 14 {1} [!0&!1&2] 24 {1}
-[!0&!1&!2] 25 {1} State: 11 [0&1&2] 8 {3} [!0&1&2] 10 {1} [!0&!1&2] 19
-{1} [!0&!1&!2] 26 {1} [!0&1&!2] 27 {1} State: 12 [0&1&2] 8 {3}
-[!0&1&2] 10 {1} [!0&!1&2] 19 {1} [!0&1&!2] 27 {1} [!0&!1&!2] 28 {1}
-State: 13 [!0&!1&2] 2 {1} [0&1&!2] 7 {1} [0&1&2] 8 {1} [!0&1&2] 10 {1}
-[0&!1&!2] 11 {1} [0&!1&2] 12 {1} [!0&!1&!2] 13 {1} [!0&1&!2] 14 {1}
-State: 14 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2] 8 {3} [!0&1&2] 10 {2}
-[0&!1&2] 12 {3} [!0&1&!2] 14 {1} [!0&!1&2] 24 {2} [!0&!1&!2] 29 {1}
-State: 15 [0&1&!2] 7 {1} [0&1&2] 8 {1} [!0&1&!2] 9 {1} [!0&1&2] 10 {1}
-[0&!1&!2] 11 {1} [0&!1&2] 12 {1} [!0&!1&!2] 15 {1} [!0&!1&2] 16 {1}
-State: 16 [0&1&!2] 7 {1} [0&1&2] 8 {1} [!0&1&2] 10 {1} [0&!1&!2] 11
-{1} [0&!1&2] 12 {1} [!0&1&!2] 14 {1} [!0&!1&2] 16 {1} [!0&!1&!2] 17
-{1} State: 17 [0&1&!2] 7 {1} [0&1&2] 8 {1} [!0&1&2] 10 {1} [0&!1&!2]
-11 {1} [0&!1&2] 12 {1} [!0&1&!2] 14 {1} [!0&!1&2] 16 {1} [!0&!1&!2] 17
-{1} State: 18 [0&1&2] 8 {3} [!0&1&2] 10 {1} [!0&!1&!2] 18 {1}
-[!0&!1&2] 19 {1} [!0&1&!2] 20 {1} State: 19 [0&1&!2] 7 {3} [0&1&2] 8
-{3} [!0&!1&2] 19 {1} [!0&!1&!2] 30 {1} [!0&1&!2] 31 {1} [!0&1&2] 32
-{1} State: 20 [0&1&2] 8 {3} [0&!1&2] 12 {1} [!0&1&!2] 20 {1} [!0&1&2]
-32 {1} [!0&!1&!2] 33 {1} [!0&!1&2] 34 {1} State: 21 [0&1&2] 8 {1}
-[!0&1&2] 10 {1} [!0&!1&!2] 18 {1} [!0&!1&2] 19 {1} [!0&1&!2] 20 {1}
-State: 22 [0&1&2] 8 {3} [!0&1&2] 10 {1} [!0&!1&2] 19 {1} [!0&1&!2] 20
-{1} [!0&!1&!2] 35 {1} State: 23 [0&!1&!2] 5 {1} [0&1&!2] 7 {1} [0&1&2]
-8 {1} [!0&1&!2] 9 {1} [!0&1&2] 10 {1} [0&!1&2] 12 {1} [!0&!1&!2] 23
-{1} [!0&!1&2] 24 {1} State: 24 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2]
-8 {3} [!0&1&2] 10 {2} [0&!1&2] 12 {3} [!0&1&!2] 14 {2} [!0&!1&2] 24
-{1} [!0&!1&!2] 25 {1} State: 25 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2]
-8 {3} [!0&1&2] 10 {2} [0&!1&2] 12 {3} [!0&1&!2] 14 {2} [!0&!1&2] 24
-{1} [!0&!1&!2] 25 {1} State: 26 [0&1&2] 8 {3} [!0&1&2] 10 {1}
-[!0&!1&2] 19 {1} [!0&!1&!2] 26 {1} [!0&1&!2] 27 {1} State: 27 [0&1&2]
-8 {3} [0&!1&2] 12 {3} [!0&1&!2] 27 {1} [!0&1&2] 32 {1} [!0&!1&!2] 36
-{1} [!0&!1&2] 37 {1} State: 28 [0&1&2] 8 {3} [!0&1&2] 10 {1} [!0&!1&2]
-19 {1} [!0&!1&!2] 26 {1} [!0&1&!2] 27 {1} State: 29 [0&!1&!2] 5 {3}
-[0&1&!2] 7 {3} [0&1&2] 8 {3} [!0&1&2] 10 {2} [0&!1&2] 12 {3} [!0&1&!2]
-14 {1} [!0&!1&2] 24 {2} [!0&!1&!2] 29 {1} State: 30 [0&1&!2] 7 {3}
-[0&1&2] 8 {3} [!0&!1&2] 19 {1} [!0&!1&!2] 30 {1} [!0&1&!2] 31 {1}
-[!0&1&2] 32 {1} State: 31 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2] 8 {3}
-[0&!1&2] 12 {3} [!0&1&!2] 31 {1} [!0&1&2] 32 {2} [!0&!1&2] 37 {2}
-[!0&!1&!2] 38 {1} State: 32 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2] 8
-{3} [0&!1&2] 12 {3} [!0&1&!2] 31 {1} [!0&1&2] 32 {2} [!0&!1&2] 37 {1}
-[!0&!1&!2] 39 {1} State: 33 [0&1&2] 8 {3} [0&!1&2] 12 {1} [!0&1&!2] 20
-{1} [!0&1&2] 32 {1} [!0&!1&!2] 33 {1} [!0&!1&2] 34 {1} State: 34
-[0&1&!2] 7 {3} [0&1&2] 8 {3} [0&!1&!2] 11 {1} [0&!1&2] 12 {1}
-[!0&1&!2] 31 {1} [!0&1&2] 32 {1} [!0&!1&2] 34 {1} [!0&!1&!2] 40 {1}
-State: 35 [0&1&2] 8 {3} [!0&1&2] 10 {1} [!0&!1&2] 19 {1} [!0&1&!2] 20
-{1} [!0&!1&!2] 35 {1} State: 36 [0&1&2] 8 {3} [0&!1&2] 12 {3}
-[!0&1&!2] 27 {1} [!0&1&2] 32 {1} [!0&!1&!2] 36 {1} [!0&!1&2] 37 {1}
-State: 37 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2] 8 {3} [0&!1&2] 12 {3}
-[!0&1&!2] 31 {2} [!0&1&2] 32 {2} [!0&!1&2] 37 {1} [!0&!1&!2] 39 {1}
-State: 38 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2] 8 {3} [0&!1&2] 12 {3}
-[!0&1&!2] 31 {1} [!0&1&2] 32 {2} [!0&!1&2] 37 {2} [!0&!1&!2] 38 {1}
-State: 39 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2] 8 {3} [0&!1&2] 12 {3}
-[!0&1&!2] 31 {2} [!0&1&2] 32 {2} [!0&!1&2] 37 {1} [!0&!1&!2] 39 {1}
-State: 40 [0&1&!2] 7 {3} [0&1&2] 8 {3} [0&!1&!2] 11 {1} [0&!1&2] 12
-{1} [!0&1&!2] 31 {1} [!0&1&2] 32 {1} [!0&!1&2] 34 {1} [!0&!1&!2] 40
-{1} --END--""")
-aa.merge_states()
-# This used to cause a segfault reported by Philipp.
-print(aa.to_str())
+    aa = spot.automaton("""
+    HOA: v1 States: 41 Start: 0 AP: 3 "allfinished" "finished_0"
+    "finished_1" acc-name: parity max odd 4 Acceptance: 4 Inf(3) | (Fin(2)
+    & (Inf(1) | Fin(0))) properties: trans-labels explicit-labels
+    trans-acc colored properties: deterministic --BODY-- State: 0
+    [!0&!1&!2] 1 {1} [!0&!1&2] 2 {1} [!0&1&!2] 3 {1} [!0&1&2] 4 {1}
+    [0&!1&!2] 5 {1} [0&!1&2] 6 {1} [0&1&!2] 7 {1} [0&1&2] 8 {1} State: 1
+    [!0&!1&!2] 1 {1} [!0&!1&2] 2 {1} [0&1&!2] 7 {1} [0&1&2] 8 {1}
+    [!0&1&!2] 9 {1} [!0&1&2] 10 {1} [0&!1&!2] 11 {1} [0&!1&2] 12 {1}
+    State: 2 [!0&!1&2] 2 {1} [0&1&!2] 7 {1} [0&1&2] 8 {1} [!0&1&2] 10 {1}
+    [0&!1&!2] 11 {1} [0&!1&2] 12 {1} [!0&!1&!2] 13 {1} [!0&1&!2] 14 {1}
+    State: 3 [0&1&!2] 7 {1} [0&1&2] 8 {1} [!0&1&!2] 9 {1} [!0&1&2] 10 {1}
+    [0&!1&!2] 11 {1} [0&!1&2] 12 {1} [!0&!1&!2] 15 {1} [!0&!1&2] 16 {1}
+    State: 4 [0&1&!2] 7 {1} [0&1&2] 8 {1} [!0&1&2] 10 {1} [0&!1&!2] 11 {1}
+    [0&!1&2] 12 {1} [!0&1&!2] 14 {1} [!0&!1&2] 16 {1} [!0&!1&!2] 17 {1}
+    State: 5 [0&1&2] 8 {3} [!0&1&2] 10 {1} [!0&!1&!2] 18 {1} [!0&!1&2] 19
+    {1} [!0&1&!2] 20 {1} State: 6 [0&1&2] 8 {1} [!0&1&2] 10 {1} [!0&!1&2]
+    19 {1} [!0&1&!2] 20 {1} [!0&!1&!2] 21 {1} State: 7 [0&1&2] 8 {3}
+    [!0&1&2] 10 {1} [!0&!1&2] 19 {1} [!0&1&!2] 20 {1} [!0&!1&!2] 22 {1}
+    State: 8 [!0&!1&!2] 5 {1} [0&1&2] 8 {3} [!0&1&2] 10 {1} [!0&!1&2] 19
+    {1} [!0&1&!2] 20 {1} State: 9 [0&!1&!2] 5 {1} [0&1&!2] 7 {1} [0&1&2] 8
+    {1} [!0&1&!2] 9 {1} [!0&1&2] 10 {1} [0&!1&2] 12 {1} [!0&!1&!2] 23 {1}
+    [!0&!1&2] 24 {1} State: 10 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2] 8
+    {3} [!0&1&2] 10 {2} [0&!1&2] 12 {3} [!0&1&!2] 14 {1} [!0&!1&2] 24 {1}
+    [!0&!1&!2] 25 {1} State: 11 [0&1&2] 8 {3} [!0&1&2] 10 {1} [!0&!1&2] 19
+    {1} [!0&!1&!2] 26 {1} [!0&1&!2] 27 {1} State: 12 [0&1&2] 8 {3}
+    [!0&1&2] 10 {1} [!0&!1&2] 19 {1} [!0&1&!2] 27 {1} [!0&!1&!2] 28 {1}
+    State: 13 [!0&!1&2] 2 {1} [0&1&!2] 7 {1} [0&1&2] 8 {1} [!0&1&2] 10 {1}
+    [0&!1&!2] 11 {1} [0&!1&2] 12 {1} [!0&!1&!2] 13 {1} [!0&1&!2] 14 {1}
+    State: 14 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2] 8 {3} [!0&1&2] 10 {2}
+    [0&!1&2] 12 {3} [!0&1&!2] 14 {1} [!0&!1&2] 24 {2} [!0&!1&!2] 29 {1}
+    State: 15 [0&1&!2] 7 {1} [0&1&2] 8 {1} [!0&1&!2] 9 {1} [!0&1&2] 10 {1}
+    [0&!1&!2] 11 {1} [0&!1&2] 12 {1} [!0&!1&!2] 15 {1} [!0&!1&2] 16 {1}
+    State: 16 [0&1&!2] 7 {1} [0&1&2] 8 {1} [!0&1&2] 10 {1} [0&!1&!2] 11
+    {1} [0&!1&2] 12 {1} [!0&1&!2] 14 {1} [!0&!1&2] 16 {1} [!0&!1&!2] 17
+    {1} State: 17 [0&1&!2] 7 {1} [0&1&2] 8 {1} [!0&1&2] 10 {1} [0&!1&!2]
+    11 {1} [0&!1&2] 12 {1} [!0&1&!2] 14 {1} [!0&!1&2] 16 {1} [!0&!1&!2] 17
+    {1} State: 18 [0&1&2] 8 {3} [!0&1&2] 10 {1} [!0&!1&!2] 18 {1}
+    [!0&!1&2] 19 {1} [!0&1&!2] 20 {1} State: 19 [0&1&!2] 7 {3} [0&1&2] 8
+    {3} [!0&!1&2] 19 {1} [!0&!1&!2] 30 {1} [!0&1&!2] 31 {1} [!0&1&2] 32
+    {1} State: 20 [0&1&2] 8 {3} [0&!1&2] 12 {1} [!0&1&!2] 20 {1} [!0&1&2]
+    32 {1} [!0&!1&!2] 33 {1} [!0&!1&2] 34 {1} State: 21 [0&1&2] 8 {1}
+    [!0&1&2] 10 {1} [!0&!1&!2] 18 {1} [!0&!1&2] 19 {1} [!0&1&!2] 20 {1}
+    State: 22 [0&1&2] 8 {3} [!0&1&2] 10 {1} [!0&!1&2] 19 {1} [!0&1&!2] 20
+    {1} [!0&!1&!2] 35 {1} State: 23 [0&!1&!2] 5 {1} [0&1&!2] 7 {1} [0&1&2]
+    8 {1} [!0&1&!2] 9 {1} [!0&1&2] 10 {1} [0&!1&2] 12 {1} [!0&!1&!2] 23
+    {1} [!0&!1&2] 24 {1} State: 24 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2]
+    8 {3} [!0&1&2] 10 {2} [0&!1&2] 12 {3} [!0&1&!2] 14 {2} [!0&!1&2] 24
+    {1} [!0&!1&!2] 25 {1} State: 25 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2]
+    8 {3} [!0&1&2] 10 {2} [0&!1&2] 12 {3} [!0&1&!2] 14 {2} [!0&!1&2] 24
+    {1} [!0&!1&!2] 25 {1} State: 26 [0&1&2] 8 {3} [!0&1&2] 10 {1}
+    [!0&!1&2] 19 {1} [!0&!1&!2] 26 {1} [!0&1&!2] 27 {1} State: 27 [0&1&2]
+    8 {3} [0&!1&2] 12 {3} [!0&1&!2] 27 {1} [!0&1&2] 32 {1} [!0&!1&!2] 36
+    {1} [!0&!1&2] 37 {1} State: 28 [0&1&2] 8 {3} [!0&1&2] 10 {1} [!0&!1&2]
+    19 {1} [!0&!1&!2] 26 {1} [!0&1&!2] 27 {1} State: 29 [0&!1&!2] 5 {3}
+    [0&1&!2] 7 {3} [0&1&2] 8 {3} [!0&1&2] 10 {2} [0&!1&2] 12 {3} [!0&1&!2]
+    14 {1} [!0&!1&2] 24 {2} [!0&!1&!2] 29 {1} State: 30 [0&1&!2] 7 {3}
+    [0&1&2] 8 {3} [!0&!1&2] 19 {1} [!0&!1&!2] 30 {1} [!0&1&!2] 31 {1}
+    [!0&1&2] 32 {1} State: 31 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2] 8 {3}
+    [0&!1&2] 12 {3} [!0&1&!2] 31 {1} [!0&1&2] 32 {2} [!0&!1&2] 37 {2}
+    [!0&!1&!2] 38 {1} State: 32 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2] 8
+    {3} [0&!1&2] 12 {3} [!0&1&!2] 31 {1} [!0&1&2] 32 {2} [!0&!1&2] 37 {1}
+    [!0&!1&!2] 39 {1} State: 33 [0&1&2] 8 {3} [0&!1&2] 12 {1} [!0&1&!2] 20
+    {1} [!0&1&2] 32 {1} [!0&!1&!2] 33 {1} [!0&!1&2] 34 {1} State: 34
+    [0&1&!2] 7 {3} [0&1&2] 8 {3} [0&!1&!2] 11 {1} [0&!1&2] 12 {1}
+    [!0&1&!2] 31 {1} [!0&1&2] 32 {1} [!0&!1&2] 34 {1} [!0&!1&!2] 40 {1}
+    State: 35 [0&1&2] 8 {3} [!0&1&2] 10 {1} [!0&!1&2] 19 {1} [!0&1&!2] 20
+    {1} [!0&!1&!2] 35 {1} State: 36 [0&1&2] 8 {3} [0&!1&2] 12 {3}
+    [!0&1&!2] 27 {1} [!0&1&2] 32 {1} [!0&!1&!2] 36 {1} [!0&!1&2] 37 {1}
+    State: 37 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2] 8 {3} [0&!1&2] 12 {3}
+    [!0&1&!2] 31 {2} [!0&1&2] 32 {2} [!0&!1&2] 37 {1} [!0&!1&!2] 39 {1}
+    State: 38 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2] 8 {3} [0&!1&2] 12 {3}
+    [!0&1&!2] 31 {1} [!0&1&2] 32 {2} [!0&!1&2] 37 {2} [!0&!1&!2] 38 {1}
+    State: 39 [0&!1&!2] 5 {3} [0&1&!2] 7 {3} [0&1&2] 8 {3} [0&!1&2] 12 {3}
+    [!0&1&!2] 31 {2} [!0&1&2] 32 {2} [!0&!1&2] 37 {1} [!0&!1&!2] 39 {1}
+    State: 40 [0&1&!2] 7 {3} [0&1&2] 8 {3} [0&!1&!2] 11 {1} [0&!1&2] 12
+    {1} [!0&1&!2] 31 {1} [!0&1&2] 32 {1} [!0&!1&2] 34 {1} [!0&!1&!2] 40
+    {1} --END--""")
+    aa.merge_states()
+    # This used to cause a segfault reported by Philipp.
+    print(aa.to_str())