diff --git a/src/Builtin/Subjects.cs b/src/Builtin/Subjects.cs
index 03b6d25..9c221c1 100644
--- a/src/Builtin/Subjects.cs
+++ b/src/Builtin/Subjects.cs
@@ -16,8 +16,8 @@
         public readonly S1 s1;
         public CombinedSubject(Builder b)
         {
-            s0 = b.Combine<S0>();
-            s1 = b.Combine<S1>();
+            s0 = b.CombineInclude<S0>();
+            s1 = b.CombineInclude<S1>();
         }
     }
 
@@ -31,9 +31,9 @@
         public readonly S2 s2;
         public CombinedSubject(Builder b)
         {
-            s0 = b.Combine<S0>();
-            s1 = b.Combine<S1>();
-            s2 = b.Combine<S2>();
+            s0 = b.CombineInclude<S0>();
+            s1 = b.CombineInclude<S1>();
+            s2 = b.CombineInclude<S2>();
         }
     }
 
@@ -49,10 +49,10 @@
         public readonly S3 s3;
         public CombinedSubject(Builder b)
         {
-            s0 = b.Combine<S0>();
-            s1 = b.Combine<S1>();
-            s2 = b.Combine<S2>();
-            s3 = b.Combine<S3>();
+            s0 = b.CombineInclude<S0>();
+            s1 = b.CombineInclude<S1>();
+            s2 = b.CombineInclude<S2>();
+            s3 = b.CombineInclude<S3>();
         }
     }
 
@@ -70,11 +70,11 @@
         public readonly S4 s4;
         public CombinedSubject(Builder b)
         {
-            s0 = b.Combine<S0>();
-            s1 = b.Combine<S1>();
-            s2 = b.Combine<S2>();
-            s3 = b.Combine<S3>();
-            s4 = b.Combine<S4>();
+            s0 = b.CombineInclude<S0>();
+            s1 = b.CombineInclude<S1>();
+            s2 = b.CombineInclude<S2>();
+            s3 = b.CombineInclude<S3>();
+            s4 = b.CombineInclude<S4>();
         }
     }
 
@@ -94,12 +94,12 @@
         public readonly S5 s5;
         public CombinedSubject(Builder b)
         {
-            s0 = b.Combine<S0>();
-            s1 = b.Combine<S1>();
-            s2 = b.Combine<S2>();
-            s3 = b.Combine<S3>();
-            s4 = b.Combine<S4>();
-            s5 = b.Combine<S5>();
+            s0 = b.CombineInclude<S0>();
+            s1 = b.CombineInclude<S1>();
+            s2 = b.CombineInclude<S2>();
+            s3 = b.CombineInclude<S3>();
+            s4 = b.CombineInclude<S4>();
+            s5 = b.CombineInclude<S5>();
         }
     }
 
diff --git a/src/EcsGroup.cs b/src/EcsGroup.cs
index 99fa224..60d1a02 100644
--- a/src/EcsGroup.cs
+++ b/src/EcsGroup.cs
@@ -71,8 +71,8 @@ namespace DCFApixels.DragonECS
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void Bake(List<int> entities) => _source.Bake(entities);
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public Span<int> ToSpan() => _source.ToSpan();
-        public Span<int> ToSpan(int start, int length) => _source.ToSpan(start, length);
+        public ReadOnlySpan<int> ToSpan() => _source.ToSpan();
+        public ReadOnlySpan<int> ToSpan(int start, int length) => _source.ToSpan(start, length);
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public int First() => _source.First();
@@ -163,7 +163,6 @@ namespace DCFApixels.DragonECS
         {
             return world.GetFreeGroup();
         }
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal EcsGroup(EcsWorld world, int denseCapacity = 64)
         {
             _source = world;
@@ -282,13 +281,13 @@ namespace DCFApixels.DragonECS
             foreach (var e in this)
                 entities.Add(e);
         }
-        public Span<int> ToSpan() => new Span<int>(_dense, 0, _count);
-        public Span<int> ToSpan(int start, int length)
+        public ReadOnlySpan<int> ToSpan() => new ReadOnlySpan<int>(_dense, 0, _count);
+        public ReadOnlySpan<int> ToSpan(int start, int length)
         {
 #if (DEBUG && !DISABLE_DEBUG) || ENABLE_DRAGONECS_ASSERT_CHEKS
             if (start + length > _count) ThrowArgumentOutOfRangeException();
 #endif
-            return new Span<int>(_dense, start, length);
+            return new ReadOnlySpan<int>(_dense, start, length);
         }
         #endregion
 
@@ -323,9 +322,9 @@ namespace DCFApixels.DragonECS
 
         /// <summary>as Intersect sets</summary>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public void AndWith(EcsReadonlyGroup group) => AndWith(group.GetGroupInternal());
+        public void IntersectWith(EcsReadonlyGroup group) => IntersectWith(group.GetGroupInternal());
         /// <summary>as Intersect sets</summary>
-        public void AndWith(EcsGroup group)
+        public void IntersectWith(EcsGroup group)
         {
 #if (DEBUG && !DISABLE_DEBUG) || ENABLE_DRAGONECS_ASSERT_CHEKS
             if (World != group.World) ThrowArgumentDifferentWorldsException();
@@ -337,9 +336,9 @@ namespace DCFApixels.DragonECS
 
         /// <summary>as Symmetric Except sets</summary>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public void XorWith(EcsReadonlyGroup group) => XorWith(group.GetGroupInternal());
+        public void SymmetricExceptWith(EcsReadonlyGroup group) => SymmetricExceptWith(group.GetGroupInternal());
         /// <summary>as Symmetric Except sets</summary>
-        public void XorWith(EcsGroup group)
+        public void SymmetricExceptWith(EcsGroup group)
         {
 #if (DEBUG && !DISABLE_DEBUG) || ENABLE_DRAGONECS_ASSERT_CHEKS
             if (_source != group.World) ThrowArgumentDifferentWorldsException();
@@ -390,7 +389,7 @@ namespace DCFApixels.DragonECS
         }
         /// <summary>as Intersect sets</summary>
         /// <returns>new group from pool</returns>
-        public static EcsGroup And(EcsGroup a, EcsGroup b)
+        public static EcsGroup Intersect(EcsGroup a, EcsGroup b)
         {
 #if (DEBUG && !DISABLE_DEBUG) || ENABLE_DRAGONECS_ASSERT_CHEKS
             if (a._source != b._source) ThrowArgumentDifferentWorldsException();
@@ -404,7 +403,7 @@ namespace DCFApixels.DragonECS
 
         /// <summary>as Symmetric Except sets</summary>
         /// <returns>new group from pool</returns>
-        public static EcsGroup Xor(EcsGroup a, EcsGroup b)
+        public static EcsGroup SymmetricExcept(EcsGroup a, EcsGroup b)
         {
 #if (DEBUG && !DISABLE_DEBUG) || ENABLE_DRAGONECS_ASSERT_CHEKS
             if (a._source != b._source) ThrowArgumentDifferentWorldsException();
diff --git a/src/EcsSubject.cs b/src/EcsSubject.cs
index 6a7d91b..30a5a41 100644
--- a/src/EcsSubject.cs
+++ b/src/EcsSubject.cs
@@ -100,8 +100,8 @@ namespace DCFApixels.DragonECS
             }
             #endregion
 
-            #region Combine
-            public TOtherSubject Combine<TOtherSubject>() where TOtherSubject : EcsSubject
+            #region Combine Include/Exclude/Optional
+            public TOtherSubject CombineInclude<TOtherSubject>() where TOtherSubject : EcsSubject
             {
                 var result = _world.GetSubject<TOtherSubject>();
                 _inc.ExceptWith(result.mask._exc);//удаляю конфликтующие ограничения
@@ -111,6 +111,20 @@ namespace DCFApixels.DragonECS
                 _exc.UnionWith(result.mask._exc);
                 return result;
             }
+            public TOtherSubject CombineExclude<TOtherSubject>() where TOtherSubject : EcsSubject
+            {
+                var result = _world.GetSubject<TOtherSubject>();
+                _inc.ExceptWith(result.mask._exc);//удаляю конфликтующие ограничения
+                _exc.ExceptWith(result.mask._inc);//удаляю конфликтующие ограничения
+
+                _inc.UnionWith(result.mask._inc);
+                _exc.UnionWith(result.mask._exc);
+                return result;
+            }
+            public TOtherSubject CombineOptional<TOtherSubject>() where TOtherSubject : EcsSubject
+            {
+                return _world.GetSubject<TOtherSubject>();
+            }
             #endregion
 
             private void End(out EcsMask mask)
@@ -120,7 +134,7 @@ namespace DCFApixels.DragonECS
                 var exc = _exc.ToArray();
                 Array.Sort(exc);
 
-                mask = new EcsMask(_world.Archetype, inc, exc);
+                mask = new EcsMask(_world.Archetype, inc, exc, _inc.Overlaps(exc));
                 _world = null;
                 _inc = null;
                 _exc = null;
@@ -170,24 +184,16 @@ namespace DCFApixels.DragonECS
         internal readonly Type _worldType;
         internal readonly int[] _inc;
         internal readonly int[] _exc;
-        public EcsMask(Type worldType, int[] inc, int[] exc)
+        internal readonly bool _isConflicting;
+
+        public EcsMask(Type worldType, int[] inc, int[] exc, bool isConflicting)
         {
             _worldType = worldType;
             _inc = inc;
             _exc = exc;
+            _isConflicting = isConflicting;
         }
 
-        public static EcsMask Union(EcsMask a, EcsMask b)
-        {
-            if(a._worldType != b._worldType) ThrowHelper.ThrowArgumentDifferentWorldsException();
-
-            HashSet<int> incset = new HashSet<int>(a._inc);
-            HashSet<int> excset = new HashSet<int>(a._exc);
-            incset.UnionWith(b._inc);
-            excset.UnionWith(b._exc);
-            return new EcsMask(a._worldType, incset.ToArray(), excset.ToArray());
-        }   
-
         #region Object
         public override string ToString() => CreateLogString(_worldType, _inc, _exc);
         #endregion
diff --git a/src/EcsWorld.cs b/src/EcsWorld.cs
index c01d4cd..adb0dfd 100644
--- a/src/EcsWorld.cs
+++ b/src/EcsWorld.cs
@@ -42,6 +42,8 @@ namespace DCFApixels.DragonECS
 
         private object[] _components;
 
+        private EcsGroup _emptyGroup;
+
         #region Properties
         public abstract Type Archetype { get; }
         public int UniqueID => uniqueID;
@@ -84,6 +86,7 @@ namespace DCFApixels.DragonECS
 
             _groups = new List<WeakReference<EcsGroup>>();
             _allEntites = GetFreeGroup();
+            _emptyGroup = GetFreeGroup();
 
             _subjects = new EcsSubject[128];
             _executors = new EcsQueryExecutor[128];
@@ -354,6 +357,7 @@ namespace DCFApixels.DragonECS
             group.Clear();
             _groupsPool.Push(group);
         }
+        internal EcsGroup GetEmptyGroup() => _emptyGroup;
         #endregion
 
         #region Debug