Working on tree optimizer, char concat flatten ok, working on alt union

StgSharp.RegularAnalysis/Abstraction/AbstractSyntaxTree.cs

@@ -152,9 +152,9 @@ namespace StgSharp.RegularAnalysis.Abstraction
                             return true;
                         }
                     case 2:
+                        _recycle.Push(_stack.Pop());
                         if (TNode.IsNullOrEmpty(cur.Node.Next))
                         {
-                            _recycle.Push(_stack.Pop());
                             if (_stack.Count == 0)
                             {
                                 _completed = true;

StgSharp.RegularAnalysis/Abstraction/ISyntaxNode.cs

@@ -66,6 +66,14 @@ namespace StgSharp.RegularAnalysis.Abstraction
              TNode nextToken
         );
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static virtual bool IsNullOrEmpty(
+                                   TNode node
+        )
+        {
+            return node is null || node.Equals(TNode.Empty);
+        }
+
         void PrependNode(
              TNode previousNode
         );